No Christian who is a chosen born-again child of God can have both a demonic spirit and the Holy Spirit living in him or her. Why is that?
That’s because God’s children receive the Holy Spirit and they do not make a practice of sinning, for God’s Son, Jesus Christ holds them securely, and the evil one, the Devil, who is the author of sin, cannot touch them! (See 1. John 2:27, 1. John 5:18)
However, Christians can have a demonic spirit influencing them in their body and mind.
That is why Christians are being told through the scriptures to humble themselves and resist the Devil so he will flee from them, by using God’s mighty weapons to knock down the strongholds of human reasoning and to destroy false arguments, bringing every rebellious thought into captivity to the obedience of Christ! (See James 4:7, 2. Corinthians 10:3-5)
This means to stay alert, stand on the Word of God in faith, and pray in the Spirit at all times! (See Ephesians 6:10-18)