Hello there!
Luckily the FAQ contains an answer to your question:
There may only be one entry per one player. This is done to support features like name editing and deleting an entry in a safe way, such that other people don't mess around with other people's entries. If you do not want this, then disable "Unique Usernames" for your leaderboard and make use of the LeaderboardCreator.ResetPlayer() function during the callback from uploading an entry. Note that after this, a player's previous entry cannot be edited nor deleted by the player themself.