Thank you for the example. Could you please have a quick look at my commands? I did collapse reads (see the top lines of the 'reads.fa' file below too). I expect that when you have multiple samples some sequences might be the same across them.
mapper.pl config.txt -d -e -p galGal4 -s processed_reads.fa -t mapped_reads.arf -h -m -i -j
Inspecting reads:
head processed_reads.fa
>ac1_0_x161356 TTTGGCAATGGTAGAACTCACACT >ac1_161356_x83226 TTTGGCAATGGTAGAACTCACA
miRDeep2.pl processed_reads.fa galGal4.fa mapped_reads.arf gga4_mirbase21_mature.fa none gga4_mirbase21_hairpin.fa