Duplex - Different lease terms on either side?

[Timothy Boyd]

Hi BP Community,

We have a duplex where we inherited the existing tenant on one side.  When we put the existing tenants on a new lease we did not charge a pet fee for their small dog and cat.  We are now screening tenants for the other side, and I wanted to put in a $25/month pet fee.  Am I allowed to charge them a pet fee if the other side (existing tenant) is not being charged for their pets?  Or would I need to have the existing tenants pay a pet fee also to make this legal?  I would plan on revising the existing tenant's lease next year to include a pet fee, but want to make sure I am on the up and up in the meantime.  Any insight is greatly appreciated!

Thanks!

Tim