1122334455667788991

This number is a prime.

                                                        112233445 5667788991

Place 1 after the concatenation of the 9 digits duplicated and obtain a 19-digit prime, whose sum of digits is the 91 appearing the end of the prime. [Silva]

(1122334455667788991, 1122334455667788997) is a sexy prime pair formed by doubling each counting digit in order and attaching at the end respectively the digits 1 and 7. [Loungrides]