Create a Quick Matching and Duplicate Rule #inSalesforce

Create a Quick Matching and Duplicate Rule #inSalesforce

Setup | Matching Rules | Use standard or create New | Object - Account | Next | Name it - like - AccountNameMatch| Account | Criteria Account Name Exact and Save

Setup | Duplicate Rules | New | Name it - like - AccountNameMatch| Enforce sharing rules | Action on Create and Edit - Block | Compare Accounts with Accounts and Matching Rule - Select the one you created | Add additional criteria as per need | Save and Activate

Test it

Create a new Account with a name that already exists and the rule will fire

Note: You can even view duplicates and merge if needed