Will I be taxed twice if I have income from multiple countries?
No, you won’t. It may be that you will have to file a tax return in more than one country, but that doesn’t mean you will be taxed twice.
States agree among themselves which state has the right to levy taxes in case of cross-border earnings. That's what they're committing to in a tax treaty. As a result, you do not pay double taxation.
Do you want to know whether your country of residence has a tax treaty with the Netherlands? And what that treaty says about taxation? Take a look at Overview of treaty countries.
What if no Dutch tax treaty exists for your country of residence?
You will never be taxed twice, even if no treaty has been concluded or ratified.
If you do not live in the Netherlands
If you do not live in the Netherlands, you must report both Dutch income and property. You will always pay tax on Dutch property in the Netherlands. You won’t pay any taxes on income which, according to the bilateral tax treaty, has been assigned to your country of residence.
If you live in the Netherlands
If you live in the Netherlands, you must report your total world income in the Netherlands.
If your foreign income, according to a tax treaty, has been assigned to that foreign country for taxation purposes, you won’t be taxed in the Netherlands for that part of your income.
If your foreign income, according to a tax treaty, has been assigned to the Netherlands for taxation purposes, you will be taxed in the Netherlands for that part of your income. Please check with the country where you have earned your income how to claim a tax relief in order to avoid double taxation.
If no tax treaty has been drawn up, you will be able to claim a Dutch double taxation relief in your Dutch tax return.