I had that error a few times and it usally shows up when the entries get doubled up.
the Delete line above does work, but as you said it would only delete the oma mobs.
if you want to delete specific rows use
DELETE FROM TBL_MONGEN WHERE FLD_ID = Insert Fld_ID Here
e.g
DELETE FROM TBL_MONGEN WHERE FLD_NAME = 1234
so just find the rows you want to delete - make sure you order by fld ID so you can see where the problem resides.
if you have a double entry - make sure you take a copy of one of the row you want to keep - when you delete the row it will remove Both rows due to them having the same Fld_ID, So take a copy, run the line and then paste the individual line back in.
if you have multiple lines affected and you end up with trouble using the SQL command, you can do as suggested and Export the Table in to Access Format, at which point you can then play around with it without the majority of restrictions (you can also then copy it from Access to Excel and run any excel commands you know to tidy it up) as long as you copy it back into Access and then Import it to SQL from an Access Source it should all work

Give me PM if you run in to problems.