How (in code) can I prevent two people starting the same crowdsourcing task at once?

I'm trying to build a Django app for a translation crowdsourcing task.

For each task in the database, I have an is_completed boolean flag that is set when the user completes the task. I also have a 'give me a random task' button, which chooses from the list of uncompleted tasks.

My question is this. How do I prevent two users being given the same task, if one user clicks the button shortly after another?

I was thinking of setting a has_started flag on the row when a task is loaded, and removing started tasks from the list of random available tasks: but what if the user starts a task and then closes the page without finishing it, so it never gets unset? I'll end up with a lot of unfinished tasks.

Could I flag this in a cleverer way with session variables that expire, perhaps? But I know it's hard to capture the 'user closes page' event reliably in JavaScript.

Thanks!

Instead of making has_started a flag, you could make it a timestamp and decide on a reasonable amount of time for task completion (which will allow you to assume that a task has been dropped after X minutes).

There is a risk that this will result in multiple translations of the same thing (i.e. if someone is really really slow and the job is recirculated early), but I think it will cover most cases.