This is a duplicate of 0006168

Thanks for linking bug 0006168, but I believe commit 71d865b is actually the cause of the problem, not the fix. The issue persists on our installation with the latest nightly (5.12.8).

Looking at the diff, the commit commented out the original regex (lines 427-428):

var currentSignature = new RegExp('(' + reNl + '){' + nlNb + '}--' + space + reNl +
currentIdentity.signature.replace(/[-[]{}()*+?.,\^$|#\s]/g, '\$&'));

and replaced it with (line 432):

var currentSignature = new RegExp('(<p>)?(<br ?\/?>( )?[ \n]?)?-- <br ?\/?>( )?[ \n]?(<\/p>)?' + currentIdentity.signature)

This replacement has two problems:

1. The regex is hardcoded to HTML patterns regardless of compose mode. The function correctly sets nl/reNl/space to different values for plain text vs HTML, but line 432 ignores those variables and always uses HTML entities (<br>,  , <p>). In plain text mode the body contains "-- \n" but the regex searches for "-- <br />" which will never match.

2. The original code escaped the signature for regex special characters with .replace(/[-[]{}()*+?.,\^$|#\s]/g, '\$&'). The replacement drops this escaping entirely and appends currentIdentity.signature raw. Any signature containing . + ( ) / ? (which is almost every signature with URLs or phone numbers) will break the regex.

The nl2 and reNl2 variables added by the commit are never used in the actual regex on line 432.

Because the regex never matches, the old signature is never removed, and the fallback paths (line 440 or 448-461) append it again, causing the duplication.

The original commented-out code was correct in approach. A proper fix should restore the mode-aware variables and the escaping, while keeping the try/catch from 0005695:

var escapedSig = currentIdentity.signature.replace(/[-[]{}()*+?.,\^$|#\s]/g, '\$&');
var currentSignature = new RegExp('(' + reNl + '){' + nlNb + '}--' + space + reNl + escapedSig);

I can reproduce this 100% of the time with plain text compose mode (SOGoMailComposeMessageType = text) and a multi-line signature containing URLs.