This can happen if a former address it being hidden by a filter. To check former address follow the steps below.
  1. Open the constituent record that is a possible duplicate
  2. Click the contact tab
  3. Under addresses check the box to see former addresses
  4. Click apply
  5. Check for the address that was listed as a possible match
  6. If they are the same constituent and the address should be active you can remove your end date
    1. Click the expand button next to address
    2. Click edit
    3. delete the end date
    4. Click save
  7. If the records are not duplicates than you can click ignore on the message