إِيمَانًا points to worship undertaken in faith, and احْتِسَابًا to seeking reward from Allah. Together, they concern the inward aim of worship, not merely its visible performance.
Example 1: Someone fasts but constantly seeks admiration for the practice; this calls for examination and improvement of intention.
Example 2: Someone fasts, prays, and gives while acknowledging personal shortcomings and hoping for Allah’s acceptance; that is the proper direction of sincerity.
Their repetition shows that the same inward purpose matters in fasting, Ramadan night prayer, and worship on the Night of Decree.