Multiple doses of antiparasitics are often indicated because of the parasite's life cycle, and the action of the drug.
In this case, cestodes (tapeworms) have a complicated life history:
-They all have indirect life cycles, meaning that when eggs are released into the environment, they must pass through several different host species before reaching the definitive or "final" host, where they are mature and can reproduce. This means that the tapeworm ova that are passed into your lizard's environment cannot reinfect him, but if he originally got the tapeworm from prey (like mice, lizards, fish, etc.) or from another parasite vector (like mites), he could still be reinfected.
-Reptiles are either the definitive host ("final" host where the parasite reproduces (asexually in cestodes)) or an intermediate host depending on the species, but either way they can cause damage to your lizard.
If the vet diagnosed tapeworm using a fecal, then there is certainly one or more adult tapeworms in the digestive tract passing mature proglottids (or ova released from proglottids). However, there can also be other immature tapeworms elsewhere in the body.
Depending on the species of tapeworm, they may live in other organs (like the liver) then migrate to the intestine to mature later. Praziquantel (Droncit) only works on adult tapeworms. So the reason for the second dose (in dogs, cats, or herps) is to kill any tapeworms that may have been in the body elsewhere at the time of the initial dose. Some vets do not prescribe multiple doses because they are certain of the tapeworm's life history, and reinfection in dogs and cats is rare (well... depends on the environment, but tapeworms are not all that common compared to other parasites).
So... personally I would go for the second dose, as recommended by your vet. If you are really concerned that the second dose could be for nothing, you can request that a second fecal exam is performed before that dose to see if there are any more tapeworms in the intestine left to treat. Either way, it is good practice to have a follow-up fecal done a few weeks to a month after the treatment to ensure everything is peachy keen. Of course if his condition does not improve or worsens, get more help sooner...!
Don't hesitate to ask questions 
Christina Miller, CAHT
www.herptiles.net
