diff --git a/src/lib/__tests__/signature-utils.test.ts b/src/lib/__tests__/signature-utils.test.ts
index b23aafca..d5b7c1b0 100644
--- a/src/lib/__tests__/signature-utils.test.ts
+++ b/src/lib/__tests__/signature-utils.test.ts
@@ -12,6 +12,7 @@ import {
   insertSignatureAboveQuote,
   signatureIdentityFor,
   signPlainTextReply,
+  signatureHasOwnDelimiter,
 } from '../signature-utils';
 
 describe('sanitizeSignatureHtml', () => {
@@ -115,3 +116,23 @@ describe('signPlainTextReply', () => {
     expect(signPlainTextReply('Thanks', 'q', null, { position: 'above_quote', separator: true })).toBe('Thanks\n\nq');
   });
 });
+
+describe('a signature with its own "-- " line', () => {
+  const own = { textSignature: '-- \nUser A\nstw-test' };
+  const plain = { textSignature: 'User A\nstw-test' };
+
+  it('is detected, also with a bare "--" or a leading blank line', () => {
+    expect(signatureHasOwnDelimiter(own)).toBe(true);
+    expect(signatureHasOwnDelimiter({ textSignature: '\n--\nUser A' })).toBe(true);
+    expect(signatureHasOwnDelimiter(plain)).toBe(false);
+    expect(signatureHasOwnDelimiter({ htmlSignature: '<p>-- </p><p>User A</p>' })).toBe(true);
+  });
+
+  it('gets no second separator in plain text or HTML', () => {
+    const delimiters = (text: string) => text.split('\n').filter((line) => /^--\s*$/.test(line)).length;
+    expect(delimiters(appendPlainTextSignature('Hi', own, { separator: true }))).toBe(1);
+    expect(appendPlainTextSignature('Hi', plain, { separator: true })).toBe('Hi\n\n-- \nUser A\nstw-test');
+    expect(buildEmbeddedSignatureHtml(own, { separator: true })).not.toContain('="separator"');
+    expect(buildEmbeddedSignatureHtml(plain, { separator: true })).toContain('="separator"');
+  });
+});
diff --git a/src/lib/compose-format.ts b/src/lib/compose-format.ts
index d1356c59..8618fe2b 100644
--- a/src/lib/compose-format.ts
+++ b/src/lib/compose-format.ts
@@ -13,7 +13,7 @@ import { htmlToPlainText } from './compose-html';
 import {
   buildEmbeddedSignatureHtml, containsEmbeddedSignature, getPlainTextSignature,
   plainTextBodyHasSignature, plainTextBodyWithoutSignature, spliceSignature,
-  type SignatureSource,
+  wantsSignatureSeparator, type SignatureSource,
 } from './signature-utils';
 
 /**
@@ -67,7 +67,9 @@ export function htmlComposeBodyToPlainText(
   const head = text.slice(0, at).replace(/\s+$/, '');
   const tail = text.slice(at + SIGNATURE_TOKEN.length).replace(/^\s+/, '');
   const signature = getPlainTextSignature(identity);
-  const withSignature = signature ? `${head}${separatorText(options.separator)}${signature}` : head;
+  const withSignature = signature
+    ? `${head}${separatorText(wantsSignatureSeparator(options.separator, identity))}${signature}`
+    : head;
   return tail ? `${withSignature}\n\n${tail}` : withSignature;
 }
 
@@ -86,7 +88,7 @@ export function plainComposeBodyToHtml(
   const signature = getPlainTextSignature(identity);
   const embedded = signature ? buildEmbeddedSignatureHtml(identity, options) : '';
   if (embedded) {
-    const marker = `${separatorText(options.separator)}${signature}`;
+    const marker = `${separatorText(wantsSignatureSeparator(options.separator, identity))}${signature}`;
     const at = text.indexOf(marker);
     if (at !== -1) {
       const before = plainTextToComposerHtml(text.slice(0, at).replace(/^\n+/, ''));
diff --git a/src/lib/signature-utils.ts b/src/lib/signature-utils.ts
index f162a940..e778037f 100644
--- a/src/lib/signature-utils.ts
+++ b/src/lib/signature-utils.ts
@@ -90,6 +90,27 @@ export function signatureIdentityFor<T extends { textSignature?: string; htmlSig
   return primary && hasSignature(primary) ? primary : null;
 }
 
+/**
+ * Whether the signature starts with its own "-- " delimiter line. Many do
+ * (imported from other clients, or set on the server), and the composer's
+ * own separator on top of it showed two delimiter lines (webmail
+ * lib/signature-delimiter.ts).
+ */
+export function signatureHasOwnDelimiter(signature?: SignatureSource | null): boolean {
+  const text = signature?.textSignature?.trim()
+    ? signature.textSignature
+    : signature?.htmlSignature?.trim()
+      ? htmlToPlainText(sanitizeSignatureHtml(signature.htmlSignature))
+      : '';
+  const firstLine = text.replace(/\r\n?/g, '\n').replace(/^\s*\n/, '').split('\n')[0];
+  return /^--[ \t ]*$/.test(firstLine);
+}
+
+/** Whether to add the "-- " separator line in front of this signature. */
+export function wantsSignatureSeparator(enabled: boolean | undefined, signature?: SignatureSource | null): boolean {
+  return enabled !== false && !signatureHasOwnDelimiter(signature);
+}
+
 export function getPlainTextSignature(signature?: SignatureSource): string {
   if (signature?.textSignature?.trim()) {
     return normalizeSignatureLineBreaks(signature.textSignature);
@@ -112,7 +133,7 @@ export function appendPlainTextSignature(
     return body;
   }
 
-  const sep = options.separator === false ? '\n\n' : '\n\n-- \n';
+  const sep = wantsSignatureSeparator(options.separator, signature) ? '\n\n-- \n' : '\n\n';
   return `${body}${sep}${plainTextSignature}`;
 }
 
@@ -186,7 +207,7 @@ export function appendHtmlSignature(
   signature?: SignatureSource,
   options: { separator?: boolean } = {},
 ): string {
-  const sep = options.separator === false ? '<br><br>' : '<br><br>-- <br>';
+  const sep = wantsSignatureSeparator(options.separator, signature) ? '<br><br>-- <br>' : '<br><br>';
 
   if (signature?.htmlSignature?.trim()) {
     return `${htmlBody}${sep}${sanitizeSignatureHtml(signature.htmlSignature)}`;
@@ -208,7 +229,7 @@ export function buildEmbeddedSignatureHtml(
   identity: SignatureSource,
   options: { separator: boolean },
 ): string {
-  const startMarker = options.separator
+  const startMarker = wantsSignatureSeparator(options.separator, identity)
     ? `<p ${SIGNATURE_RANGE_MARKER}="separator">-- </p>`
     : `<p ${SIGNATURE_RANGE_MARKER}="start"></p>`;
   const endMarker = `<p ${SIGNATURE_RANGE_MARKER}="end"><br></p>`;
diff --git a/src/screens/ComposeScreen.tsx b/src/screens/ComposeScreen.tsx
index 59263f4a..42b881fa 100644
--- a/src/screens/ComposeScreen.tsx
+++ b/src/screens/ComposeScreen.tsx
@@ -67,7 +67,7 @@ import { shouldBlockEditorRemoteImages } from '../lib/editor-html';
 import {
   signatureIdentityFor, buildEmbeddedSignatureHtml, containsEmbeddedSignature, spliceSignature,
   insertSignatureAboveQuote, getPlainTextSignature, appendPlainTextSignature,
-  plainTextBodyHasSignature, plainTextBodyWithoutSignature, SIGNATURE_RANGE_MARKER,
+  plainTextBodyHasSignature, plainTextBodyWithoutSignature, SIGNATURE_RANGE_MARKER, wantsSignatureSeparator,
 } from '../lib/signature-utils';
 import {
   getAutoFilledPlaceholders, getPlaceholdersFromTemplate, substitutePlaceholders, templateBodyToHtml,
@@ -1083,7 +1083,7 @@ export default function ComposeScreen({ route, navigation }: Props) {
   const placeSignaturePlain = React.useCallback((body: string, identity: Identity): string => {
     const sig = getPlainTextSignature(identity);
     if (!sig) return body;
-    const sep = signatureSeparatorEnabled ? '\n\n-- \n' : '\n\n';
+    const sep = wantsSignatureSeparator(signatureSeparatorEnabled, identity) ? '\n\n-- \n' : '\n\n';
     if (isReplyLike && signaturePosition === 'above_quote') {
       const idx = body.indexOf('\n\n');
       const head = idx === -1 ? body : body.slice(0, idx);
@@ -1139,7 +1139,7 @@ export default function ComposeScreen({ route, navigation }: Props) {
           stripped = plainTextBodyWithoutSignature(body, prev);
         } else if (prev) {
           const old = getPlainTextSignature(prev);
-          const marker = `${signatureSeparatorEnabled ? '\n\n-- \n' : '\n\n'}${old}`;
+          const marker = `${wantsSignatureSeparator(signatureSeparatorEnabled, prev) ? '\n\n-- \n' : '\n\n'}${old}`;
           if (old && body.includes(marker)) stripped = body.replace(marker, '');
         }
         return signatureIdentity ? placeSignaturePlain(stripped, signatureIdentity) : stripped;