Problem description & analysis:
Table A-C has multiple rows of data, where certain rows are empty; and column C has duplicate values. Values of column E are unique; they correspond to certain values of column C, but there are inconsistent letter cases among the corresponding values.

We need to align table A-C to column E and ignore the case difference.

Solution:
Use SPL XLL to do this:

=spl("=?1.align@a(E@1(?2).(upper(~)),upper(~3)).conj()",A2:C7,E2:E4)

As shown in the picture below:

Explanation:
align@a function aligns a sequence to another sequence and performs grouping. conj() function concatenates members of all groups. E@1 converts a multilayer sequence to a single-layer one.