Usernames getting modified – numeral "1" being added

[david (David Taylor)]

@tnorthcutt

will Discourse let you create a group with the name of an existing user, and append 1 to the username

No, you will get “Group name is not available”.

We had a bug with the validation a few months ago, which might be why you ended up with this situation. The bug is now fixed, so it shouldn’t happen in the future.