You might think that if you've coloured your hair before, you're obviously not allergic to dye. But an allergic reaction can happen at any time, regardless of how often you've had your hair coloured. Even if you're using a product you've used several times before.
Which is why it is recommended that you should do a patch test every time.
The risk may be small, but if you suffer a sudden, severe reaction, not only could it damage your health, it could sensitise your system to the point that you can never have your hair coloured again.
And if you're using a permanent home hair colour that needs mixing, make sure it's in exactly the same proportions as will be applied in full measure to your scalp, as the slightest difference could change how your skin will react.