Because INFJs often feel so misunderstood and alien in the world, partnering with someone who understands them so well (just as they understand others) is likely to be a source of great comfort and friendship.
Dating for the INFJ personality type can feel forced and awkward.
And they often feel undervalued, having to fight hard to get their piece of the proverbial pie and do it authentically.
This is hard enough on its own, but can be particularly demoralizing if an INFJ gets the sense that someone out there (in this case, the INFJ partner in the immediate vicinity) is already doing the job.
A final word of caution: part and parcel of that concept of competition in the relationship is the potential for one or both partners to question his/her value in the world, particularly if they are constantly around another INFJ.
INFJs already intuitively know that, generally speaking, the world undervalues their special brand of know-how.