To give a full answer, we must define what type of conjugation we're doing. You've said second person singular, but there are three (!) different pronouns that can refer to singular things in the second person: tú, usted, and the oft forgotten vos.
If the intended pronoun/person is usted, then we use the third-person singular conjugation and there's no question there.
If the intended pronoun/person is tú, then there is categorically no -s. The only endings for tú in the preterite are -aste and -iste.
However, if the intended pronoun/person is vos, things get more complicated. The verbal forms for vos never coallesced into a single panhispanic conjugation (actually, more technically, they split off from a single one, but that's a bit beside the point). As a result, the standard conjugations for vos vary by region and register, and what is correct in one circumstance is wrong in another. The following are forms that are standard in one or more regions according to the Nueva Gramática Española (ASALE/RAE):
- hablar: hablasteis, hablastes, hablaste, hablates, hablate
- comer: comisteis, comistes, comiste, comites, comite
- vivir: vivisteis, vivistes, visiste, vivites, vivite
The forms that end in -steis are exclusive to the vos reverencial. The forms that end in -ste are generally just tuteante forms that have fallen into the vos paradigm, although in some cases they are in fact derived from the -steis form, both monophthongizing and losing the -s.
If you just hear someone say something like ¿Ya comistes hoy?, they may be completely wrong, but if they are a voseante speaker, then depending on their particular vos paradigm, they may be perfectly correct.