Learn Portuguese (Portugal): Deram-lhe o prémio a ela, não a ti.

Yes, it is grammatically redundant, but it is extremely common in Portuguese. This is known as clitic doubling. The indirect object pronoun -lhe already means 'to her', but adding a ela explicitly clarifies who is receiving the action and adds strong emphasis. It mimics the vocal stress in the English translation: 'They gave the prize to HER.'

Could I just say Deram o prémio a ela without the -lhe?

Grammatically you can, but it sounds much less natural in European Portuguese. When you use an explicit indirect object starting with a (like a ela or ao João), native speakers almost always include the corresponding indirect object pronoun (-lhe or -lhes) attached to the verb.

Why is it a ti at the end instead of just não te?

Because it follows the preposition a. When a pronoun is the object of a preposition (like a, para, de, or com), you must use prepositional pronouns. For informal singular 'you', that pronoun is ti. You cannot use the unstressed object pronoun te directly after a preposition.

Since there is no subject, does Deram strictly mean a specific 'They' gave the prize?