So why and were does it state that i cant put duplicate weapons on vehicles with multiple hardpoints or crew slots?
Hardpoints/crew
Page 64 of the RRG
What an aggressive way to ask a question.
Why has a simple answer: Because that's how FFG wrote the rules.
As for why they wrote that rule, my assumption is it has to do with FFG then only having to include a single part for each hardpoint/crew upgrade to enable players to make their models WYSIWYG.