The first part:

Code:

{ read A; read B; printf "%s\n" "$A" "$B";  sort; } < file

reads the first line from file into the shell variable A (read A) and the second line from file into the shell variable B (read B), reprints those two lines unchanged (printf '%s\n' "$A" "$B" and sorts the remaining lines from file (sort). This sorts the data in file while keeping the headers (unsorted) at the start of the output.

All of the output from the first part is then piped (|) into the second part:

Code:

awk '!T[$1]++'

which creates an array indexed by the first field on each input line (T[$1]) and sets the value of that array to the number of times that index has been seen (after using the previous value of the variable). In awk, statements consist of a condition section and an action section. In this case the condition is !T[$1]++ and there is no action section. When there is no action section, the default action (print the line) is performed if, and only if, the condition evaluates to a non-zero numeric value or a non-empty string value. Since undefined elements of an array are treated as a zero or an empty string (depending on context), the condition !T[$1]++ is evaluated as follows the first time a line is seen with a given string in the first field:and since the condition's value is non-zero, the line is printed. On subsequent lines with the same first field the evaluation is:and since the condition's value is zero, the line is not printed.

Note that the sort in the first part grouped all lines with the same 1st field together and (if both an "add" and a "delete" appeared for that value, the line(s) with "add" for that 1st field value will come before any line(s) with "delete".

Nice, simple, straight-forward magic. The only likely problem with this approach is that if the data in your file contains a 1st field value that is identical to the 1st field in one of the header lines, those data lines will be lost. But, as long as you don't have a user named "user", that shouldn't be a problem for you.