Short answer: no, dyeing your hair twice in one day is not a good idea. It means double chemical processing in a single day, and hair professionals advise against it, especially with bleach or permanent dye.
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Why dyeing twice in one day is risky
Each application of dye is a chemical process. Doing it twice in one day significantly increases the risk of dryness, breakage and scalp irritation. Hair that has just been coloured has already been put under stress, and a second application adds more stress before it has had any time to recover.
Dye cannot strengthen hair
Claims that dyeing is harmless, or that it strengthens hair, are not true. Dye is cosmetic. The visible hair shaft is dead tissue, so nothing applied to it can make it stronger in a living sense.
An invented fading claim
Some articles invent an effect that supposedly makes a second dye job fade faster because the first application is somehow remembered by the hair. No such effect exists, and it should not be used to decide when to colour your hair again.
What to do if your colour goes wrong
If a colour result goes wrong, safer routes are to wait before re-colouring, to use a colour remover made for that purpose, or to see a professional colourist. These options are safer than immediately applying a second dye on the same day.
Before you colour
Always do a patch test and a strand test before colouring, following the product instructions. This helps you check for a reaction and see how your hair responds before you apply dye to your whole head.
This article is general information only and is not medical advice. Please consult a healthcare professional for personal advice.