How can one overcome the dullness of habit that weakens devotion when repeatedly reciting a specific Surah or making the same supplications?

Habit is a neutral concept. It is good for a person to accustom themselves to good deeds and to be steadfast in their performance without affectation; good is a habit. Among the greatest causes for a servant's rectitude is for them to have acts of obedience that they regularly perform, preserve, and accustom themselves to. This was also the Prophet Muhammad's, peace be upon him, consistent and steadfast practice. As for "habit" in the sense of not feeling the act of worship and performing it mechanically, devoid of spirit, this is a danger that must be heeded, as the reward is linked to the heart's presence in worship. What helps achieve the heart's presence in worship includes: varying the acts of worship, patiently maintaining the intention, striving in supplication, not abandoning the act, preparing oneself for worship, and avoiding what distracts the heart.