Recently Squadhelp has stopped accepting domain names that have won sometime in the 7 year? history of Squadhelp, whereas previously it just used to inform us if a name had won before and it was up to our discretion to enter it or not.
Keeping in mind a few points
1] Domain names expire or are abandoned regularly
2] Available domain names can still be registered by general public outside Squadhelp
I feel an available domain name should be able to be entered into contests even if it has won sometime previously after a grace period, maybe even up to a year (Standard registration period for a URL).