Duplicate Characters

Usually when you create a character with a name already in use it gives you an error saying the name is taken. And for the most part that usually works. But in the 4+ years ive been running my server, at least 4 different times someone was able to register a name that already existed. I was wondering if anyone else experienced this and if there are any clues as to what could be the cause and how to fix it.

Both accs had the charinfo file, each different. But only one charsave file exists. Both accounts are able to use it.

edit: ok it seems both players created the character in the main menu but did not enter a game server with it. Basically just both creating it to save the name for later use and then they both wanted to play that name in the new season. So my guess now is that when u create a character in the main menu, it only creates the charinfo file and leaves the charsave empty but the Name checker checks charsave probably. At least thats my current theory but im not sure 100% sure on everything yet. Would still appreciate any info u guys have.