More advanced 'find' command?

Hi folks, not sure if I'm in the correct place. I am looking for assistance to accomplish the following, send me away if needed, sorry:

Find directories with more than one file with a .mkv extension in it

I have a large media collection (6k+ movies), however I suspect Plex has mismatched some files, double-downloaded etc., so to start, I'd like to clean up multiple accidental downloads in the same directory. It's ok if there is a .mkv with a .srt (subtitle) file as an example, but multiple files with the same extensions (mkv) in this case is what I'm after. Going through this 1-by-1 is impossible. My server runs unraid for the time being, which is slackware based. I'm noob'ish with cli, pretty basic guy. Any help or direction where I can get help with this command is awesome, thanks folks.