dnd 5e: Does the sorcerer's extended spell metamagic option extend the duration of a spell exponentially or linearly?

When you mention that you can spend 1 witchcraft point to double its duration, up to a maximum of 24 hours, does that mean that every time you spend one witchcraft point you add the original duration again? Or does it exponentially double the duration?

For example, if I spend 4 sorcery points on a 1-minute spell, does it become 5 minutes (1 minute duration added four times) or 16 minutes duration (1 minute duplicate four times)?

What I meant by this question is to only use the extended spell on a spell once, but to use more than 1 sorcery point according to the metamagic description.

I know you can't re-extend a spell if it's already active, but I'm talking about the initial cast, where you can spend multiple sorcery points to double the duration of the spell.