a mim/a ti/a ele/a ela

While it might seem redundant, this 'doubling' serves vital purposes in Portuguese:

1. Emphasis and Contrast
By adding the prepositional phrase, you highlight the person involved. It often implies 'to me and not to someone else.' This pattern can reinforce both indirect objects (dative) and direct objects (accusative). When used with a direct object, it acts as a redundant (or pleonastic) direct object. Note that when tonic pronouns (mim, ti, ele, ela, nós, vós, eles, elas) act as direct objects, they must take the preposition 'a', which explains why a direct object uses a preposition here. With an indirect object, it acts as a redundant indirect object.
Example: Ele viu-a a ela, mas não a mim. (He saw her, but not me.)

2. Fronting (Topicalization)
In neutral word order, the prepositional phrase goes at the very end of the clause. However, for heavy emphasis, the prepositional phrase is often moved to the beginning of the sentence. Note that in European Portuguese, fronting the prepositional phrase does not automatically trigger proclisis (e.g., A mim parece-me bem.).
Example: A mim parece-me bem. (To me, it seems fine.)

3. Disambiguation (Clearing up Confusion)
The indirect object pronoun lhe can mean 'to him,' 'to her,' or 'to you (formal).' Adding a ele, a ela, or a você makes it clear who you are talking about.
Example: Eu entreguei-lhe a chave a ela. (I delivered the key to her.)

4. Reflexive Reinforcement
This pattern is also frequently used with reflexive pronouns to emphasize that the action is done to oneself, often adding words like mesmo or própria.
Example: Ele enganou-se a si mesmo. (He fooled himself.)
Warning: For the 3rd person, you must use a si and not a ele/a ela when the action is reflexive. English speakers often mistakenly say Ele enganou-se a ele mesmo due to interference from 'himself/herself'. While this might be heard in informal Brazilian Portuguese, it is grammatically incorrect in standard European Portuguese.

Regional Usage Note:
Clitic doubling is a hallmark of everyday European Portuguese. In Brazilian Portuguese, it is largely absent and mostly restricted to the 1st person (a mim) for heavy emphasis. Brazilians typically drop the clitic entirely for the 3rd person rather than doubling it.

Grammar Rules:

• Mandatory Clitic in European Portuguese: If you use a tonic pronoun as a direct or indirect object (like a mim, a ti, a ele, a ela), the corresponding clitic pronoun (me, te, o/a/lhe) is mandatory (this does not apply to directional complements or certain prepositional verbs). For example, Ele viu a mim is ungrammatical in EP; it must be Ele viu-me a mim. However, in Brazilian Portuguese, dropping the clitic and using the prepositional object alone (Ele viu a mim) is grammatical and a standard way to express contrastive focus.
• Clitic-to-Tonic Mapping: The clitic must match the prepositional phrase: me ➔ a mim, te ➔ a ti, se ➔ a si, o/a ➔ a ele/a ela/a você, lhe ➔ a ele/a ela/a você, nos ➔ a nós, vos ➔ a vós, os/as ➔ a eles/a elas/a vocês, lhes ➔ a eles/a elas/a vocês. Remember that the clitic and the tonic pronoun must agree in gender and number (e.g., Ele viu-a a ela; Eu vi-os a eles).
• Animacy Restriction: Clitic doubling with a ele / a ela is almost exclusively used for animate objects (people and sometimes animals). It is generally not used to refer to inanimate objects.
• No Crase: The preposition a never takes a crase (grave accent) before personal pronouns in this pattern. It is always a ela, a ele, a você, a mim, a ti, never à ela.
• The standard reinforcement requires the preposition a. While you might hear doubling with para in colloquial Brazilian Portuguese, it is considered grammatically incorrect (a syntactic error or an error of regência). Usually, if Brazilians use para mim, they drop the clitic entirely rather than doubling it. In fact, when dropping the clitic for indirect objects, Brazilians overwhelmingly prefer the preposition para over a in everyday speech (e.g., saying Eu entreguei a chave para ela instead of a ela).
• It uses 'tonic' pronouns: mim, ti, você, ele, ela, si, nós, vós, vocês, eles, elas.
• Note on 'si': In European Portuguese, a si is often used as the emphatic form for formal 'você'. In Brazilian Portuguese, si is almost exclusively reflexive (meaning 'himself/herself'). BP learners should use a você for emphasis.
• In European Portuguese, the clitic usually stays in enclisis, while in Brazilian Portuguese, proclisis is more common. Furthermore, in casual Brazilian Portuguese, the clitic is often dropped entirely (e.g., Eu vi ela) rather than doubled.