As you cannot have more than one record with the same Constituent ID in the Raiser's Edge, this exception indicates that you have attempted to add a new constituent to the Raiser's Edge with a Constituent ID that already exists on a record in the system. How you resolve this will depend upon several factors:
1) If the Constituent ID is the correct way of identifying the constituent in RE for the constituent in your import, ensure that you have a Criteria Set in place to match records to the Constituent ID (this is a default setting in Importacular, but it can be removed by end users).
2) If the Constituent ID is NOT the correct way of identifying the constituent in RE for the constituent in your import, you most likely should remove the field that you have mapped to Constituent ID in that mapping area.
3) Ensure that you did not change the record from Update to Create in the Review Screen when the match was based upon a Constituent ID.