When replying a mail, the signature duplicates. If the signature was already on the original mail, it is moved to the top.

Settings:
    Reply style: Inline (Outlook style)
    Add a Signature
    Assign signature as default
    Signature above original message

Steps to reproduce:
    Have someone send you a mail.
    Click on "Reply".
    The signature appears double one after the other.
    Send that reply.
    Have your friend reply your mail.
    Reply your friend's new mail.
    The signature appears three times, and both of the signatures of the previous email are removed.
    Continue sending replys and watch your signatures accumulate at the top. :)

It seems that all of your signatures from the inline reply are moved to the top, but that's not the whole problem since when you first reply a mail that doesn't has your signature on it, you still get two signatures.

PS. I didn't find any similar bug logged.