We have received very few reports noting an issue where the HTML signature will repeat multiple times (even if it is only pasted once). The issue seems to occur when switching between different accounts. This is a documented iOS / Apple Mail bug, see: this discussion for more details, or this report, or this one.
Please also check if it may be related to your HTML code (or pasting your code more than once). You can try one of the free HTML templates we have listed.
For now, we suggest you contact Apple or visit the Mac Store to get further assistance with this issue.