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So much interest in this topic, let's try another way for performance since his file is huge. This will edit the file in place and should be blazing fast.

linux$ cat t.c
#include <stdlib.h>...

Alternatively:
awk '/^[THD]/{print p;p=$0;next}{p=p FS $0}END{print p}' infile
or
awk '/^[THD]/{print RS}1' ORS= infile

you can give a try with this
awk '{n="\n"}/^HEADER/{print;delete A[D];next}/^D/{p=(!A[D]++)?z:n;printf p $0;next}{y=(/^TRAILER/)?n:z;printf y $0 y}' tst

Awk solution:

awk '
NR==1{print}
/^TRAILER/{if(a) print a; print ; exit}
NR>1 && !/^D/{print a" "$0; a=""}
/^D/{ if(a) print a; a=$0 }' infile

Hi... a solution using awk
awk '{
if(NR == 1)
print $0;
else if(NR != 1)
{
if($0 !~ /^TRAILER/)
{
if($0 ~ /^D/)
{
printf "%s\n", $0 ; next
}
else
{
}
}
else
{
last=$0;
}

Here you have a perl script, I hope it works now

$ cat infile
HEADER474687
D1356jkl ugbliuybikb 879870
898976098 9687680
D77656757 uhgliug liygoiygig
D98679hjh kjbgihguygfu ugliyh...