Allow me to put on my Cassandra Campbell "official toy reviewer" hat on
--(AVN Novelty Magazine, Playboy Radio Toy Review Girl)
Unfortunately, the novelty business isn't regulated and often times manufacturers will use a lesser grade of material to save on costs. The synthetic material will break down with wear, types of lube, and time. Most double ended dongs are made of "jelly" as the toy industry folk call it, a rubber/plastic blend that is designed to be bendable and durable. You can also find rubber or latex dildos. Often times it's not the dildo itself, but the treatment used to protect the item if it's heat sealed when packaging, that can cause irritation.
If the item feels different from when you got it originally (usually it gets softer, then it starts to get sticky and may or may not have a slightly stronger scent)
If you are looking for alternatives to material, I suggest looking for silicone or cyberskin for bendable ones, and acrylic or glass for rigid dongs.
Try something like this:
Majestic CyberSkin Double Cock
What brand and type (water based, silicone) lube are you using and how are you cleaning and storing your toys?