This one's easy to debug.
Open up your table in design view then select "Indexes" from the "View" pulldown menu.
If your table has a primary key, it will be annotated with a key symbol immediately beside the associated index name. If the primary key consists of more than one field, each of those fields will have a key symbol beside it. Take note of all the "Field Names" which are part of that primary key.
The next possible source is that you have a unique index ... this is different than a primary key, but, prevents duplication as do primary keys.
To verify if any unique indexes exist, view indexes for your table and click on each row with an "Index name" (except for the primary key you already noted in the step above). If the field entitled "Unique" has a value of "Yes", write down that field name and any other field names immediately below the current record that has a blank "Index Name" ... that means that the next field or fields are part of the index which did have and "Index name". The field or fields for each of these indexed names form a unique index.
Once you understand which combinations of fields form primary or unique indexes, you'll understand exactly why the new record you're trying to enter from your form is violating your tables index rules and generating the duplicate notification.
Does this help?
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