Learn Portuguese (Portugal): O professor está a escrever a história.

Use está a escrever (is writing) to show that the action is happening right now, at this exact moment. Using just escreve usually means 'writes' in a general sense or as a regular habit.

Why is there an a before escrever?

In European Portuguese, to say you are currently doing something, you use a form of the verb estar (to be) + a + the action verb in its original, unchanged form. Together, está a escrever creates the meaning 'is writing'.

The word a appears twice in this sentence. Do they mean the same thing?

No, they serve completely different purposes! The first a is a connecting word (preposition) used to build the 'is writing' action. The second a right before história is the feminine word for 'the'.

Could I use é instead of está since they both mean 'is'?

No, because estar is the only verb you can use to build this 'happening right now' structure. We use está for ongoing, temporary actions. The other verb for 'to be' (ser, which gives us é) is used for permanent facts, like stating someone's profession.