The time form deployed as part of the Matter Management application has extra processing built into the save handler that may be impacted when user organizations have a large number of background Process Builders/Flows active on the time or matter objects, thus allowing the Save button to momentarily become enabled for a second or more. Selecting the Save button again during that elapsed time will generate a duplicate time entry.

Why is there a duplicate to the time entry I just created?

Users will need to be trained to select the Save button only once when creating time.