In four-part harmony only the root tone can be doubled in dominant seventh chord ("Guide to harmony" - Tchaikovsky). Why, what is the reasoning behind it? I am interested only from classical harmony perspective (!)
For the vast majority of harmonic rules I always find some explanation/motivation and try not to study them as just a set of recipes. In this case I seem to fail.
Disclaimer: Yes, I understand that all these rules are constantly broken even during Common Practice Period.