The mobile "Add discussion" button helpfully adds ~~~~ automatically to the end of your comment. Unfortunately, since editing a talk page section to reply to a thread doesn't do this, you still get into the habit of adding ~~~~ manually, which via force of habit leads to duplicate sigs whenever you use "Add discussion". I suggest either removing this automatic sig, or detecting "~~~~" at the end of the text and not adding it again if present.
- Auto-add signature to replies. If "~~~~" is inside markup do not append.
Use the beta cluster for this one.
- Make sure you are logged in and visit https://en.m.wikipedia.beta.wmflabs.org/wiki/Talk
- Click talk button at bottom of page
- Click add discussion in top right of overlay
- Write some text.
- click save in top right
- After the save has completed, in the talk overlay click your new discussion. Verify your username is inside the talk page section
- type in the reply box and hit save
- Verify your username is in the reply
- Run the above steps again but this time put "~~~~" (signature wikitext) somewhere inside your text. Ensure that your username doesn't appear twice.