Do I need another 'which' in this sentence?

I'm trying to make the sentence below sound better:

As a first year undergraduate, I am keen to explore possible careers which, I think, would challenge me and (which) would allow me to create positive change in the world.'

Do I need another which before would allow me to create...? Or is it optional?

The second which is optional, but the first which should be a that because it's restricting "possible careers" [Grammarist]. You might want to consider rewriting your sentence as follows, sticking with your same words:

As a first year undergraduate, I am keen to explore possible careers that, I think, would challenge me, and allow me to create positive change in the world.

Note: I've revised in response to comments from Jack and alephzero.

The second "which" is definitely optional, and leaving it out makes the sentence much crisper too.

Drawing an analogy to a mathematical equation in the form of x(a+b) = xa + xb, here's how the sentence construction looks:

"... I am keen to explore possible careers which, I think, (would challenge me + would allow me to create positive change in the world)". So "which i think" automatically applies to both "would challenge me" and "allow me to create...".

P.S: For that matter, the 2nd "would" can also be taken out. It's all a matter of writing style.

My opinion is that the sentence should look like the following:

As a first-year undergraduate, I am keen to explore possible careers which, I think, would challenge me and allow me to create positive change in the world.

I would probably change "keen" to "excited." You might also want to consider writing the sentence like the following:

As a first-year undergraduate, I am keen to explore possible careers that will challenge me and enable me to create positive changes in the world.

Omit "I think" as it makes your statement sound less confident.

"As a first year undergraduate, I am keen to explore possible careers which, would challenge me; enabling me to create positive change in the world."

Hope this helps :)