How much more will the heavenly Father give the HOLY SPIRIT to those who ask him!

[tonychanyt]

Luke 11:

9 And I tell you, ask, and it will be given to you; seek, and you will find; knock, and it will be opened to you. 10 For everyone who asks receives, and the one who seeks finds, and to the one who knocks it will be opened. 11 What father among you, if his son asks for a fish, will instead of a fish give him a serpent; 12 or if he asks for an egg, will give him a scorpion? 13 If you then, who are evil, know how to give good gifts to your children, how much more will the heavenly Father give the Holy Spirit to those who ask him!”

Is the promise of the Holy Spirit being made here for believers or unbelievers?

Believers. Luke 11:

11 What father among you, if his son asks for a fish, will instead of a fish give him a serpent;

The son asks his father. Metaphorically, a believer asks the heavenly Father.

If you then, who are evil, know how to give

διδόναι Present Infinitive Active, repetitive actions

good gifts to your children, how much more will the heavenly Father give the Holy Spirit to those who ask him!”

If the promise is for believers, wouldn't that contradict the fact that believers, by definition, already have the Holy Spirit?

The promise is related to repetitive (daily) giving. It is not referring to the unique giving of the indwelling Holy Spirit (Paraclete). It refers to the repetitive filling of the Holy Spirit. It happened in Acts 2:

4 All of them were filled with the Holy Spirit and began to speak in other tongues as the Spirit enabled them.

It was a specific enablement.

It happened again in Acts 7:

55 But Stephen, full of the Holy Spirit, looked up to heaven and saw the glory of God, and Jesus standing at the right hand of God.

Colossians 1:

9 For this reason, since the day we heard about you, we have not stopped praying for you. We continually ask God to fill you with the knowledge of his will through all the wisdom and understanding that the Spirit gives,

The Spirit continually gives wisdom to believers.

Paul commanded believers in Ephesians 5:

18 Do not get drunk on wine, which leads to debauchery. Instead, be filled with the Spirit,

This was not once and for all.

be filled
πληροῦσθε (plērousthe)
Verb - Present Imperative Middle or Passive - 2nd Person Plural

Is Luke 11:9-13 a promise for believers or unbelievers?

For believers' daily spiritual enablement and development. In fact, I pray to be filled with the Holy Spirit daily knowing that I have already had the indwelling Spirit living inside me (my human spirit) since some decades ago