It has been reported to Alacritty that in certain circumstances the Paste key is sent multiple times to the winit application.

The relevant bug report in Alacritty can be found here:
alacritty/alacritty#1842

Based on the troubleshooting there, it looks like the key event is actually sent twice, however user-related problems probably can't completely be excluded since there has been noone able to reproduce this issue.

My initial thought was that it might be related to key repeat rate, but that seems to be default.