Generally, it works like this (IANAL): when the country one is working with has the necessary legislation regarding double taxation, the tax paid in the country one is working in, is deducted from the Polish tax. Most often, when people leave Poland to live and work in an EU country, they will pay full tax in the EU country they live in and have zero taxable income in Poland. This means one does not even have to file a tax return in Poland. When I left Poland in 2006, the legislation was not in place yet.

I think, can't remember exactly, the legislation was introduced in 2007, I never had to pay any taxes in Poland while working in other EU countries, I filed a zero tax return in Poland only once, as the taxes have been paid in the other EU country.

There are edge cases where one still falls under double taxation but they are regulated by respective country's tax treaties. Details in Polish: https://europa.eu/youreurope/citizens/work/taxes/double-taxa..., list of treaties per country: https://ec.europa.eu/taxation_customs/individuals/personal-t....