Learn Spanish (Latin America): Todo lo que acumulaste lo vamos a etiquetar hoy. - Everything that you accumulated, we are going to label today.

This is the core of topicalization. Because the direct object (Todo lo que acumulaste) was moved to the very front of the sentence for emphasis, Spanish requires you to double it with a matching pronoun. The lo refers right back to the things you accumulated.

Could that second lo be attached to the end of etiquetar instead?

Yes, it absolutely can. Because etiquetar is an infinitive verb, you can attach the pronoun directly to the end of it. Saying Todo lo que acumulaste vamos a etiquetarlo hoy is perfectly correct and means exactly the same thing.

Why do we need to say Todo lo que instead of just Todo que for 'everything that'?

In Spanish, you cannot place que directly after todo to mean 'everything that.' You must bridge them with lo, forming lo que, which acts as a neutral 'that which.' So, Todo lo que literally translates closer to 'All that which.'

Why is acumulaste in the past indicative and not the subjunctive?