orthography - Is the possessive of "one" spelled "ones" or "one's"?

I've been confused about this as long as I can remember. Should it be:

One should do ones duty.

or

One should do one's duty.

I'm guessing it should be the latter. But that doesn't sit well with the possessive pronoun 'its'. For example:

It is its own purpose.

vs.

It is it's own purpose.

Here, the former seems clearly correct.

Answer

The correct answer is one's!

All possessives get an apostrophe, except the standard possessive pronouns and these are:

yours, his, hers, ours, theirs, its

Apart of these, always add an apostrophe.