toLowerCase on TextInput value is creating duplicate text if capital letter created

Unfortunately this is an issue that has been open for a couple of years with no solution, you can check this thread, no one found a solution. There is a temporary workaround until the React Native team fixes this bug as it seems to be taking too long, check it out here.

add these three lines inside TextInput, it should fix the problem, original answer source

 autoCapitalize="none"
 secureTextEntry={true}
 keyboardType={"visible-password"}

see my this answer for example