[Recommendation]Pelagus: Add a warning message when trying to add an existing account

[booleksandra]

Currently, when you try to add an existing account to your wallet, the system accepts it and notifies you of the success of adding the account

Preconditions:

• Install and log in to the extension Steps to reproduce:
  1. Go to the “Select an account” page
  2. Click on [+ Add Quai account].
  3. Click on [Import from Private Key].
  4. In the window that opens, fill in the "Paste private key string" field with valid data ( 0xa775ebd01fdc824a22ccfadd6faea485b3f97949df5155987f82a7548dc20d03 )
  5. Click [Import account].
  6. Repeat all the steps from the beginning in the direct sequence (1, 2, 3, 4, 5)

Expected result: When trying to fill in the “Paste private key string” field with the data of an account that already exists in the wallet, a warning message appears and the user cannot add this account