To find duplcate records in a column use Select duplicate records by this field... which is accessible by right clicking the column header in the Table view then from Tools sub-menu choose Select duplicate records by this field..."

I'm talking about creating a field in the the model so that I can create a filter to exclude / include them as needed.

Greg - If you insert one or more De-duplicate Operations blocks into your DataManager workspace data flow, you can branch records into different flows/destinations based on the number of duplicates. You can choose to create an additional field giving the number of duplicates for each specific field and run additional tests on this added unique value count field if necessary.

Following on from above post, you can do it with two de-duplicate blocks for this case where you use one to filter all the unique first record and a second to get the duplicate records. The Source field in this case is your field for filtering on.