It seems to me (YMMV . caveat above) that splitting duped chars by a variety of little used chars (or, maybe most frequently used chars) has certain merit... because it makes usage-frequency testing slightly more complex and may thus throw off any brute force using frequency testing.

#!/usr/bin/perl
#!/usr/bin/perl
use strict;
use warnings;
use 5.012;

# See 910956.pl (re playfair cipher) for salt, key and encoding -- # 
as a follow on, modify encoding with possibility of multiple 
# (i,e. "varying" or "inconsistent") insertions between dup letters

my @range = ("Q","V","X","Y","Z",);
my $seen = "";

my $message = uc(<>);
$message =~ s/[^A-Z0-9]//gi;                # remove punct, etc
$message =~ s/j/i/gi;
$message =~ s/q/O/gi;                      # replace [Qq] with [Oo]
$message =~ s/\s+//g;                      # remove spaces
$message .= "X" if length($message)%2==1;     # make length an even va
+lue

my @char = split('', $message);

say "\n" . "-" x10 . "\n";

for my $char(@char) {
    if ($seen eq ($char)) {
        my $i = int(rand(4));
        my $letter = $range[$i];
        print $letter . $char;        # recode as push to encoded arra
+y for later printing
    } else {                              # as five char groups
        print ($char);     }             # recode: ditto
    $seen = $char;
}

say "\n";
[download]

The tested I/O includes (with apologies for the pre tags below)

Quoth the Raven, 'Nevermore!' ...or three (3) or four (4) words to that generally accepted effect.
----------
OUOTHTHERAVENXNEVERMOREORTHREXE3ORFOUR4WORDSTOTHATGENERALXLYACVCEPTEDEFYFECTX
^            ^         ^    ^^^^^    ^^^                ^^^^ ^^^     ^^^^   ^

# obviously, this has limitations:

The SECRET stash is located cinq feet below ground at 41.7N, 73.4W. Its key is X734ab#@17ffyj.
----------
THESECRETSTASHISLOCATEDCINOFEYETBELOWGROUNDAT417N734WITSKEYISX734AB17FVFYi
# meaning determinable from context          ^^^ ^^^
# but the new key is corrupted...                                ^^^ ^^^ ^^