Dyeing hair twice in one day can cause hair damage and dryness. The first dyeing process opens up the hair cuticle, allowing the second dye to penetrate more deeply into the hair and cause more damage. It is also possible for the two dyes to interact, resulting in an unexpected and undesirable colour. To avoid this, it is best to dye hair once in a single session and wait a few days before dyeing it again. Additionally, using a deep conditioner after the second dyeing can help to restore hair's natural moisture and protect it from further damage.