assalam wa alykum
what should be done if a person sometimes cannot pronouns the dua because of the slip of tongue and sometimes pronouns it correctly ? can the person continue saying that dua or should not recite it ?

All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad, sallallaahu ‘alayhi wa sallam, is His Slave and Messenger.

Undoubtedly, it is preferable for the Muslim to pronounce the words of the supplication properly as reported in Arabic if possible. If he/she is not able to say it properly for some difficulty in pronunciation, however, there will be no blame on him/her, and he/she should not abandon it for that reason. Allaah The Almighty Says (what means): {Allaah Does not Charge a soul except [with that within] its capacity.}[Quran 2:286]

It is permissible for a non-Arab to supplicate Allaah in a foreign language as previously clarified in Fatwa 83636. However, the Muslim should learn Arabic, since it is the language of the Quran and helps the Muslim understand the texts of Sharee’ah properly. For further information, see Fatwa 81468.

Allaah Knows best.