Right click the account that shows in the folder pane on the right. select settings. then follow the instructions here https://support.mozilla.org/en-US/kb/Signatures

I get the error message An account. With this name already exist. Please enter a different account name.

over on the left hand side of Thunderboird is a list of folder one of with is called "Local Folders;

Right click it and select settings. Change the account name to something different. Add a one perhaps as you have obviously added the same account more than once and the thing can not proceed with duplicate account names..

You've offered no solutions here and the instructions you link to DO NOT work. I have version 52.2.1 and no matter how you change the options to place the signature, THEY DO NOT WORK. There are NUMEROUS other threads about this issue dating back several years and Thunderbird has YET to acknowledge the problem OR more likely, doesn't give a rat's *ss what it's user think.

Bnick said

You've offered no solutions here and the instructions you link to DO NOT work. I have version 52.2.1 and no matter how you change the options to place the signature, THEY DO NOT WORK. There are NUMEROUS other threads about this issue dating back several years and Thunderbird has YET to acknowledge the problem OR more likely, doesn't give a rat's *ss what it's user think.

If you want help  that is not the way to get it.  I will lock this thread as it is not your.  If you want support ask your own question. 

I will lock this topic as it is not yours and you issue in no way resembles that of this topic.

I consider what I have received in the email following your diatribe spam.