Instruments without frets don't have this problem. I played violin for many years. When you play a double stop (two strings at the same time), since there are no frets, you can play true 3rds, 6ths etc. The harmony is so "pure" that it causes a third harmonic to ring (which is how you know you're doing it right). My violin teacher always insisted that e-flat and d-sharp are not the same. When you're playing in different keys you have to put your finger in a slightly different place.