Live in duplex, separate water heater when neighbor uses wash machine my pressure goes down, why?

Please don't duplicate post, someone(volunteer!) will get to you. Pressure probably drops because duplex is supplied by a single source water line. Do you have a separate meter? City water or well?

One meter, city water.
Sorry new here

I agree with ma, you share the same main and that washer may have a big draw. How old is the building? You could ask the owner or the folks in the other unit to trottle down their supply valves to their washer, it will only add a few minutes to their wash time.

There is no doubt that both systems are connected. Who Pays the water bill? How old is the building and what material are the water pipes? Copper, galvanized or plastic? The problems is more about volume than anything else. What you need is a separate system for each duplex. Let me know how you make out. Good luck, Tom