awk '
    /once was/ { animal = $5 }
    /he ate/   { print $4, animal } ' |
sort -u |
awk '
    NR == 1 { prev = $1; printf "%s %s", $1, $2 }
    NR >  1 { if (prev == $1) { printf " %s", $2 }
              else { prev = $1; printf "\n%s %s", $1, $2 } }
    END     { printf "\n" } ' |
awk '
    { if (NR > 1) { print "" }
      print "Food:", $1, "Animals who ate it:", NF-1
      print "========"
      for (N=2; N<=NF; N++) { print $N } } '

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VGhlcmUgb25jZSB3YXMgYSBEb2cKCldlZG5lc2RheSBoZSBhdGUgQXBwbGVzIApUaHVyc2RheSBoZSBhdGUgQXBwbGVzCkZyaWRheSBoZSBhdGUgQXBwbGVzClNhdHVyZGF5IGhlIGF0ZSBjYXJyb3RzCgpUaGVyZSBvbmNlIHdhcyBhIEJlYXIKClR1ZXNkYXkgaGUgYXRlIGNhcnJvdHMKV2VkbmVzZGF5IGhlIGF0ZSBjYXJyb3RzClRodXJzZGF5IGhlIGF0ZSBjaGlja2Vu

There once was a Dog

Wednesday he ate Apples 
Thursday he ate Apples
Friday he ate Apples
Saturday he ate carrots

There once was a Bear

Tuesday he ate carrots
Wednesday he ate carrots
Thursday he ate chicken