Here is the trick: create a relation to self, use Match Conditions to select the fields for duplicates finding. Then add a Summary column to calculate # of records (duplicates). When you have more than one, you have duplicates. Check the settings of the Duplicate Contacts relation for details.
To prevent duplicate entries, you should create a validation rule with the condition [Duplicate Count] is greater than 0. Moreover, you have to add the additional Match Condition Id<>Id to the relation. Due to this match condition the system will allow to modify existing records and save changes. The example is adjusted in the Companies table.
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