I could get debugging info:

+ for p in '$partitions'
+ addinfo=           
+ fs=                
+ mountpoint=        
+ '[' -n '' ']'      
+ '[' -n '' ']'      
+ '[' -z '' ']'
+ check_for_label
+ '[' -x /lib/udev/vol_id ']'
++ /lib/udev/vol_id /dev/sdc1
+ PARTINFO='ID_FS_USAGE=filesystem
ID_FS_TYPE=vfat
ID_FS_VERSION=FAT32
ID_FS_UUID=4862-215E
ID_FS_UUID_ENC=4862-215E
ID_FS_LABEL=MIKAUSB
ID_FS_LABEL_ENC=MIKAUSB'
++ awk -F= '/ID_FS_LABEL=/ {print $2}'
++ /lib/udev/vol_id /dev/sdc1
+ label=MIKAUSB
+ case $label in
+ case $PARTINFO in
+ '[' -n MIKAUSB ']'
+ part=LABEL=MIKAUSB
+ addinfo=' # /dev/sdc1'
++ awk '-F ' '{s[$NF]++}END{for(i in s)if(s[i]>1)print s[i], "labels " i}' /tmp/
tmp.yOtRM0YvOG
+ warn='2 labels GRMLCFG'
++ awk '{print $3}'
++ echo 2 labels GRMLCFG
+ expr GRMLCFG = MIKAUSB
+ '[' -z '' -a -z 1 ']'
+ fs=auto
++ uniq
++ fstype /dev/sdc1
+ scanfs=vfat
+ '[' -n vfat ']'
+ fs=vfat
+ '[' vfat = swap ']'
+ '[' vfat = auto ']'
+ echo LABEL=MIKAUSB '' 'vfat # /dev/sdc1'
LABEL=MIKAUSB  vfat # /dev/sdc1

Looks like the reason for the problem is the duplicated LABEL.