Can you try typing 4202 in the search box and see what result the program pulls up for Field Data? If there is more than one entry, click on the one(s) other than the one on W-2 and check whether that's the cause of the problem. This should be your first course of action before anything else (if you have not already done so).
If you can't find a duplicate entry using the search box, it means that you don't have a direct entry for the exact amount (and I can see that you're confident that you wouldn't have made separate entries that would total the same amount elsewhere), you can then test your theory of duplication by doing the following -
1) Remove the $4,202 from your entry for State pickup amount {MD} on the W-2 input screen and see whether that will eliminate both Line 3 and Line 5. If this is, indeed, a duplication, this should eliminate the amounts on both lines; and
2) Change the $4,202 from your entry for State pickup amount {MD} on the W-2 input screen to some other amounts and see whether it changes both Line 3 and Line 5. Again, if this is from a duplication, changes on both lines should synchronized.
If #1 eliminates Line 3 but Line 5 sticks, it may mean that the system went haywire, flowed that "duplication" to Line 5 on the backend, and that "duplication" can no longer be removed from the frontend. #2 should then give us more clues for what happened.
If #2 changes both Line 3 and Line 5, that should be a clear indication that there is a "duplication".
If #2 changes Line 3 but Line 5 shows a summation of $4,202 and the new amount you entered for this experiment, that would be an indication that not only is there a "duplication" but that it is a persistent error that will accumulate with each change.
Once #2 proves that there is a duplication, I'd suggest that you delete and re-create the existing W-2. If that fixes your problem, it'd be more expedient than contacting Support - unless you have more faith in them than me (and many others here). Otherwise, you will need to contact Support to have them look into this for you.
As you can see from the input and output screens below, this is not a systemic issue in ProConnect Tax:
