Delete lines containing repeated text

These ^@ are null characters, which have an ASCII code of 0.

You can delete them using:

tr -d '\000' < myfile > myfile.out

or:

sed 's/\x0//g' < myfile > myfile.out

It's possible that this is a file hole. I have also seen this issue in the past - these null characters appeared in my logs when I was running out of disk space and processes were trying to write to them.

This is standard procedure for awk

awk '
{
  k=$2
  for (i=3;i<=NF;i++)
    k=k " " $i
  if (! a[$1])
    a[$1]=k
  else
    a[$1]=a[$1] "<br>" k
}
END{
  for (i in a)
    print i "\t" a[i]
}' long.text.file

If file is sorted by first word in line the script can be more simple

awk '
{
  if($1==k)
    printf("%s","<br>")
  else {
    if(NR!=1)
      print ""
    printf("%s\t",$1)
  }
  for(i=2;i<NF;i++)
    printf("%s ",$i)
  printf("%s",$NF)
  k=$1
}
END{
print ""
}' long.text.file

Or just bash

unset n
while read -r word definition
do
    if [ "$last" = "$word" ]
    then
        printf "<br>%s" "$definition"
    else 
        if [ "$n" ]
        then
            echo
        else
            n=1
        fi
        printf "%s\t%s" "$word" "$definition"
        last="$word"
     fi
done < long.text.file
echo