Remove Duplicate Messages

Z 3 z 5 hwěžkow pohódnoćeny

wot Wužiwar Firefox bc0820 dnja Now. 22, 2016 · trajny wotkaz

Hello!
I have searched for hours and could not find a solution for this. I have an IMAP Gmail account, and my "All Mail" folder has 50146 messages. I run the extension and it works fine, it searches and finds about 13000 duplicates. However, when I click to delete the duplicates, it does nothing. I can notice that the number of messages in the folder is still the same (50146).
Is there a reason for this? Is this because the "All Mail" Gmail folder is undeletable, or maybe because the folder is too large?? Please help!!!