In case of third person (both singular and plural), the indirect object is normally duplicated. This is probably due to so-called “la a personal” (read more on espanolavanzado).
This means that, in sentences like I gave Marie the book, we would use both le and a Marie:
LE { di ~ he dado } el libro a Marie.
The same is true in case of an object in plural
Tienes que decirLE(S) la verdad a tus padres.
It is usually optional, although you will see this duplication 99.999% of the time, and the lack of this dummy le might come as unnatural/literary or simply incorrect to many natives.
In case you really want to master the redundancy and know when it’s obligatory and when it’s optional, here’s a small guide that covers (probably) all the cases.
The use of le with object in plural
Apart from its redundancy, the pronoun le doesn’t really convey any information, and so many people (in fact standard practice, although discouraged in careful language) use le instead of les with objects in plural:
Tienes que decirLE(S) la verdad a tus padres.
You can use the singular form especially when the explicit object (the one introduced by a) is right after the verb:
Tienes que decirLE(S) la verdad a tus padres.
A tus padres tienes que decirLES la verdad. [here the singular form le may sound odd]
Obligatory duplication
In certain cases, the duplication is unavoidable. These cases occur when:
- The explicit object is another pronoun [a él, a ellas etc.]
- The explicit object goes before the verb.
- With verbs like agradar, encantar, gustar and similar.*
- With verbos de afección psíquica, like animar, asustar, molestar etc.*
Points (3) and (4) are no longer valid if the object is an indefinite pronoun like todo(s), nada, algunos, and such.
Examples:
1. «LE dije eso a él.»
2. «A Juan LE han denegado la beca.»
3. «La película no LE agradó mucho a Núria.»
4. «No LE motiva a Sebas el sistema de puntuación.»
5. «La actuación no (LES) agradó a todos»
Source: §5.2a, DPD