VerifyListener Android

We use a verifyListener on a TextField. With the verifyListener we catch all letters because its a number field. When the user enters a letter the verifyListener opens a ClientDialog and deletes the letter. That's great. But when the user enters again somethig worng the verifyListener checks the textField a few times an opens the ClientDialog twice. And when the user enters tree times a letter the verifyListener checks again a few times and opens the ClientDialog also twice. That problem only occures on Android. On iOS the verifyListener checks the textFied once per enter no matter how often we entered a letter. How can we open the clientDialog always once?