Not necessarily. You can't get duplicate affixes on one item, unless you have a mythstone equivalent of that affix.
For example, you can't have 2× 200% WD on one weapon but there is a way to add extra WD% on a 200% WD weapon. Using the 50% WD Endow mythstone on a 200% WD weapon will work and it basically means 250% WD.
You can also do this on an item with resource cost, crushing blow, deadly strike, weaken. If a mythstone use the same affix as the crystal or epic affix, you can have duplicate affix.
So you can't have epic gold find and crystal gold find at the same item or epic 15% block on an item that already has 45% block.
No duplicate affix is actually quite good in making smarter builds and duplicate affixes only possible if you have a mythstone of that affix.
All sets, then add a elixir mythstone on one item works. The same with MP Absorb on a legend MP absorb item , extra attack and a mythstone with extra attack, multi attack with a multi attack mythstone, crushing blow mythstone on a crushing blow item , etc. Mythstones are extremely useful in that regard apart from their affixes.