So your Value List derives from the contents of a field in the same table ? I suggest you set up a layout with only that field (make sure that it's an edit box and set wide) and scroll through the records - you may find that some fields have more than one entry if it has previously been used as a source field for a checkbox set. You can then locate the 'dodgy' ones and fix them.
Most of the time it is a minor difference or typing style that does creates a near duplicate.
NOTE: To correct the problem you must change one or more of the past records. I added a COPY and PASTE button on each record in the "Correcting" List type layout to speed up the correcting process. ie, Copy the right version and Paste in all the wrong ones to replace them.
So I created a new table for just that field of the most common items, built the value list from that new Table. You can still add Edit... and check them from time to time.
One trick that I have used when setting up a value list to draw values from the same table that was being editied by the user is to add a single value checkbox to the right of the field that enters a 1 in this "include" field. I then define a calculation field as:
If ( Include ; ValueField )
And based my value list on this calculation field instead of ValueField.
This produces a small sub set of my total records that actually supply values to my value list. The main reason for that was I had a situation where I had about a half dozen commonly used values and a nearly unlimited number of names that might be used only once or very rarely, but I found that it also worked well for managing data entry errors that might appear in the value list. To review my selected values, I just performed a find for the records where the Include check box was selected and then reviewed them for errors, removal, etc...
It is basically the same, but picking and choosing is still a labor of love. Using a separte table and Edit.. makes the User think and perhaps a Help note of explanation will help them be more accurate.
Note that regardless of which table serves as the value source, finding and correcting an error in it would go like this:
Enter FInd mode in a layout based on the value source table.
Paste/enter/select the incorrect value (You can format this field with the same value list to make this easy to do.)
perform the find.
Edit the field to correct the error
Use Replace Field contents to adjust the value in all other found records to match your edited value if your find pulled up more than one record.