I have resque and resque-scheduler workers with 2 different queues,
they do the same task, fetch links for a certain website and save that links.
What will happen if resque-scheduler and resque workers work in parallel and do the same task (fetching links for the same website)?
How can i handle such situations?
Either you have not clarified your setup or there are some big issues there. Resque and Resque-scheduler were meant to be run together. Resue-scheduler is only supposed to schedule tasks in the future. Such tasks are still executed by Resque workers. Please read this section on their homepage on github: https://github.com/resque/resque-scheduler#delayed-jobs. To quote them,
This will store the job ... in the resque delayed queue at which time the scheduler process will pull it from the delayed queue and put it in the appropriate work queue for the given job and it will be processed as soon as a worker is available (just like any other resque job).
So, there you go. Keep running your resque workers and schedulers together forever. To answer the other part of your question, if you schedule some task through scheduler and the same task is also queued for resque to pick up directly, the net outcome depends on the task execution logic. "Fetching something from a website" sounds a harmless thing to do twice. But if you update some transaction table to make payments to your vendors based on the result of the fetch, you are in deep trouble.