how to possible allow duplicate values in primary key column?

Answer: No it is not possible.

The wording Primary Key imply non duplicate values, it is by design !

You can have duplicate values in Non Primary Key, it is the only way.

As stated in Solution 1, you cannot have actual duplicate values in the primary key.
HOWEVER, it may be possible to have primary keys which appear to be duplicates but aren't. For example, if the primary key was textual (nvarchar) then the following could happen:
ABC
ABC 
 ABC
The first two differ by a trailing space on the 2nd value. The 3rd has a leading space. If you don't look carefully at the actual values, they may appear to be duplicates.

Actually IT IS POSSIBLE, in my database I have a FORM linked to TABLE 1, containing a SUBFORM linked to TABLE 2.

TABLES 1 and 2 are linked by a primary key which does not allow duplicates.

Now my FORM is showing TWO RECORDS with same primary key...I can see them in the form, but there is no trace of the duplicate in the TABLES.

Of course I cannot delete the record in TABLE 1, because then both duplicates disappear from FORM.
And if I do, then input again the record... reappears the duplicate in the FORM.

It's a miracle...I don't really know how I manage to do it.

Anyone could help out?