To merge the constituents successfully, change your merge process or settings so that the constituent assigned as a staffing resources is the target in the merge. You can identify this constituent because they will also have the Staff constituency. 

If you are merging two constituents: 
  1. Go to Constituents > Duplicates
  2. Click Merge two Constituents 
  3. Under Target, select the constituent that is assigned as the staffing resource. 
  4. Under Source, select the other constituent. 
  5. Click Merge. 
If you are merging constituents within a mass merge process: 
  1. Go to Constituents > Duplicates 
  2. Click on Constituent Merge 
  3. Click on the hyperlinked name of your merge process
  4. Click Edit process
  5. Change the Target constituent criteria so that the target constituent is the constituent that is assigned as the staff resource.