Does the employer need to ensure that a foreign employee has the right to work in Finland even if he/she already has a Finnish tax number?

Yes. The employer must ensure that a foreign employee has the right to work in Finland, and he must keep this information at the workplace. Official transactions with the authorities (e.g. the Tax Administration, KELA, a local register office or an employment and economic development office) do not constitute proof of the right to work in Finland.