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cat f1.txt

a 455
b 443
c 6655
d 554
e 6

$ cat f2.txt

a 56
b 5
x 678

awk '$0 !~ /#/{arr[$1]=arr[$1] " " $2}END{for(i in arr)print i,arr[i]}' f1.txt f2.txt

x 678
a 455 56
b ...