Inspired from How could a memory (Tom Riddle) have performed Avada Kedavra?, is Avada Kedavra the only way to kill a wizard in a one-on-one duel?
Based on what we've seen in the books, no, Avada Kedavra is not the only way to kill a wizard (or for that matter a witch or muggle).
There are spells that directly cause death: Sectumsempra, for instance. There are also spells that can be used to cause death, though that is not the primary use: Incendio, Wingardium Leviosa (lift a guy and drop him). There are charms that can influence the surroundings to cause death: lifting a boulder and dropping it on somebody's head will do some damage. And finally, the Imperius Curse can influence someone else to bring about their own death (or used to make a third party murder someone).
However, these spells can be rectified. A skilled wizard, when caught on fire, can likely put themselves out. Snape reverses Sectumsempra and the Imperius Curse can be fought.
It seems, therefore, that Avada Kedavra is almost a surefire way. I say almost because Harry survived (special case, I know) and Dumbledore blocked the spell once -- with this, though, it wasn't a one on one and based on the description Harry gives when the curse is used on him it does not appear there is time to react. Indeed, it seems Avada Kedevra is inescapable on a one-to-one duel unless you're Harry potter.
To summarise: No, it's not the only way, but is the least possible to negate the effect.
EDIT: As Xantec pointed out, Molly's curse against Bellatrix was an instant kill, meaning Avada Kedavra is not the only spell that cannot be repelled or in some way negated, assuming, as he said, Molly wouldn't use that spell: it's not in character, plus the spell wasn't identified and if it was the familiar flash of Green Light it's likely it would have been named. This means there are instant-kill spells that aren't Unforgivable: the Ministry should get to work, me thinks.
Well, I figure being hit straight in the chest with a blasting curse or Dolohov's Purple fire wouldn't be so good for your life span. There must be dozens of lethal curses we never heard about in the canon. No need for sectum sempra; a simple Diffindo at the throat would be deadly:
"D-diffindo, " she said, pointing her wand at Ron, who roared in pain as she slashed open the knee of his jeans, leaving a deep cut.
A spell violent enough could crush your skull, or stop your heart. AK is simply the fastest and most efficient way to kill with magic. As fanfiction writers like to call it, there must be the whole branch of "Battle Transfiguration" spells to conjure knives, arrows, and suchlike.
I assume it is not the only way, i could imagine a spell like sectumsempra could also be fatal depending on where it landed. But it certainly is the most direct method. being that magic is flexible i would bet based on the creativity of a Torquemada-esk evil wizard there would be many ways to kill another wizard in a duel.
There are many ways a person can be killed, but as Mac Cooper points out, many can be deflected or blocked.
Many have already been mentioned in other answers, but there are a few additions I thought I could bring to the table. Also worth noting, most any spell, aimed at vital areas such as the head, heart, or neck, could be fatal.
Sectumsempra- This could be the most effective on the list barring the fact that it is unknown to the wider wizarding community.
Diffindo-In Deathly Hallows, Hermione cuts Ron's knee. Aimed at the neck, for instance, could cause death.
Petrificus Totalus/Stupefy- While both spells on their own were mentioned in above answers, I haven't yet seen the combo. Based on How was this character killed without Avada Kedavra? some of the answers suggested it was this combination that killed Bellatrix Lestrange. Even if it wasn't her cause of death, it certainly is lethal.
Antonin Dolohov's Spell-
Dolohov made a slashing motion with his wand, purple fire flew out, and Hermione have a little, "oh" before collapsing" ...She was pale and looked as though dead. It would be all his fault if they were hurt. He, Harry, had lured his friends here, it would be his fault if one of them died,"
For the sake of the length of my answer, Madam Pomfrey also mentions that it could have been deadly, so while unidentified, this could be a spell for killing.
I tried my very best to not include any duplicates of other answers, but please let me know if there are duplicate ideas.
Diffindo, if aimed at the heart or head, could kill a wizard/witch, but can be reversed (possibly) by Vulnera Sanentur, as can Sectumsempra. Petrificus Totalus could kill somebody (causing them to fall from a great height). But, remember, Avada Kedavra didn't kill Harry Potter, because it was blocked by the simplest charm, love. So The Killing Curse doesn't always kill people...