In Spanish there are two (partially overlapping) sets of object pronouns: the unstressed (átonos), clitic pronouns, and the stressed (tónicos), independent pronouns. The unstressed object pronouns are me, te, la/lo/le, nos, os, las/los/les, se. The stressed object pronouns are mí, ti/vos, él/ella/ello, nosotros, vosotros, ustedes, ellos/ellas, sí (those in bold appear only in some dialects). The stressed object pronouns are the ones that you have to use after prepositions.
A first or second person direct object (i.e. I, you, we) can appear as an unstressed pronoun: "Te he perdido." This is enough for a neutral statement. However, you can also use a stressed pronoun to emphasize or to show some contrast or resolve an ambiguity. In this case, the stressed pronoun will be ti and it will be preceded by a because that's how you mark a personal direct object. But if you include a stressed direct object pronoun, you have to keep the other (unstressed) one in place, even if it's redundant. So you say "Te he perdido a ti." You will only do this to emphasize who has been lost (yes, I have lost you) or to contrast with an alternative (I have lost you, not someone else, not just anyone, but you).
Sometimes speakers will use this emphatic form for no apparent reason. It's not wrong; it's supposed to show something. You can also find things like these in songs, because popular composers often pad their lyrics to conform with the metric or to make them rhyme.