Error Message when using percentage slider to complete a task

When in a project if I complete a task, that has an auto approval process on it, by just putting 100 into the percent complete box and hitting enter, it works.

However some team members have been completing tasks another way that started causing an error message to pop up.

Error message says "Error Duplicate key value violates unique constraint".

They are going into the task (w/the auto process) and then clicking on the "percentage slider" button and moving it to 100%. It still works but this message is popping up now and they are constantly having to close the error message.