Session IDs not unique?

Our site is pretty frequently getting this error:

Cannot insert duplicate key row in object 'dbo.sessions' with unique index 'sessions_id_unique'.
The duplicate key value is (7dd02e829601a87b563db65c4748385c84544106). [20018] (severity 14) [(null)]'
in /path/to/site/vendor/laravel/framework/src/Illuminate/Database/Connection.php:381

is this a normal occurrence or is there something configured wrong?