TheRBLXalts is one of the most effective bots that will clean your server from alternate accounts. It is currently in development but has a large set of commands and functionality, which are the following:

/bugreport - Reports a bug directly to developers
/findaltaccounts - Shows all alternate accounts in the guild
/findbannedaltaccounts - Shows users in the guild evading bans
/invite - Links to invite the bot to your server!
/setup - Setup the bot in your server with one slash command
/subscribe - Choose a channel where you want to receive important messages about bot's status & maintenance
/verificationbypass - Bypass a member's verification. Note that this  only is for server moderators
/verify - Verify yourself
/verifymember - Force-verify a member. Note that this is only for server moderators

Voting for TheRBLXalts Verification