The interesting part: KeePass allows the pasting of a password -- only once. Subsequent attempts to paste the same content are fruitless.
How does KeyPass manage to know when I've used the password?
Keepass has an auto-clear function. The clipboard can be auto-clear after xx seconds. If the option is activated you will be able to paste several times the same clipboard (password, username, url, etc).
To adjust the timer go on Tools > Security tab : "clipboard auto-clear times (second)":