I think my water bill is being split 50/50 with the upstairs unit. What can i do?

Just to immediately make it clear why this would be such an issue, the upstairs unit in our duplex has 4 people living there. I live alone. If its being split 50/50 that means i’m paying a significant portion of their bill.

For context I just moved into this duplex so this is the first bill I have received. I got the keys a month ago but I just moved in a few days ago. Meaning the house was sitting empty with basically zero water use during the usage period. The bill that came for that timeframe of no use, was higher than any bill at my old house that had two ppl showering and using water daily. So thats an issue.

I have emailed the landlord but probably wont hear back till Monday and I cant get the issue out of my head so I was hoping to hear some of your thoughts.

Are we thinking the bill is so high bc they’re splitting the upstairs units use with me? I really cant think of any of explanation other than a leak but theres no visible water in the basement or anywhere else.

If they are splitting it 50/50 between the two units how can i fight that bc it’s absolutely not fair when theyre using wayyyyyy more water than me and have 4 times the amount of people. I already use less water than 1 normal person, now put it against 4 ppl and the amount extra id be paying for them is exorbitant and not something I budgeted for.