Water bill DOUBLES at separetely metered duplex

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Hi everyone, i just got my water bill on a separately metered duplex and it jumped from $115 per side to $230 per side for a total of $460 this bill...Im wondering what could be causing this. Its unlikely both units have running toilets. I just bought the property 6 months ago so this is only my second bill (quarterly billing). Could the water department be playing catchup on my bill? What should i do any thoughts?