There are six kinds of 2 out of 4 gift combinations, namely AB, AC, AD, BC, BD and CD.

Among the gifts selected by any two customers.

Suppose one person's gift is AB, and ask the second person to have exactly the same product as the first person.

The second person knows all four things except AB (both are the same) and CD (both are different).

In this case, the probability that only one variety is the same is 4/6=2/3.

Similarly, if the first person's gift is a combination other than AB, the probability of just having a product of the same variety is also 2/3.

The probability that only one species is the same is 2/3.