There are two kinds of double negatives:
I do not 'not like' her, but she annoys me sometimes
It is not unusual ....
These can exist in standard English and Polish. The two negatives (both nie) mean almost but not quite the same thing as a positive.
I ain't seen nothing
Don't tell no-one
These are non-standard in English. The meaning is negative.
English-speakers may see the Polish - Niczego nie widziałem - as a double negative, and therefore 'strange'. I was taught that 'nie' establishes the negativity, which 'ni' only emphasizes.
However, we seem to be wandering from the thread!