NO they aren't. taxation of the same income in multiple states is quite common.
just a few examples assuming both states have an income tax
rental income - property located in state X taxpayer domiciled in state Y. taxed in both states
business income - earned in state X taxpayer domiciled in state Y. taxed in both states.
wages earned in one state taxpayer domiciled in another- unless there is a reciprocal agreement between the states, taxpayer pays taxes in both states.
in these situations, the state of domicile usually allows a credit for taxes paid on the same income in the non-resident state.
.
