multiple team members work on a single task associated to a story in the sprint backlog

Why not celebrate the occasion, and find ways to reinforce this ability to collaborate and to limit work in progress?

Thanks for your response Ian, It is in my opinion that when we have more pending tasks in no status, it will be good use of time for team members to work on different tasks as well as for accountability during the daily stand-ups as against multiple developers working on a single tasks. 

I definitely will celebrate this collaborative practice when there are less items in the sprint backlog in pending status.

Completely agree with @Ian Mitchell.  Celebrate and evangilize the occasion.  This is the premise of pair programming in action. I'll also give you an example of why to celebrate this from a "management" perspective.  Every time two people work together you are minimizing risk associated to the change. It also makes it easier for a team to self-organize around work when multiple people are familiar with large parts of the code base.  It will also help improve your estimations because there are fewer parts of the code base that are unknown. 

And this is not something you celebrate only when there are few items in the Sprint Backlog.  This is something you celebrate and recognize every time it happens and encourage to be done all the time.

I would suggest looking into the concepts of pairing and mobbing. There's plenty of experiences out there, but I'd agree that this isn't something to be discouraged. The one thing that I would encourage when using these techniques, however, is to consider them in planning. This is especially true when the team is first experimenting with these techniques. It may take a while to figure out how to effectively pair and mob on the work, which may result in less work getting done. But once the effective techniques are learned by the team, I suspect you'll see a number of benefits.

in my opinion

That may be the first issue, is that the role of Servent Leadership?

Do you have evidence of a real issue (proving your opinion is not just guts feeling ?)

What is the problem you what to make transparent ?

This is a good way to limit the work in progress. besides, it also allows getting each sprint backlog item done, being done throughout the sprint. In this manner, the team will be, more likely, in the right way to deliver a releasable increment at the end of the sprint. In addition, It presents many opportunities for the team members to learn from each other and develop their ability to work together. Bearing in mind, the necessity to inspect and adapt each technique to the context.

I definitely will celebrate this collaborative practice when there are less items in the sprint backlog in pending status.

You might be doing more harm than good. Which role in Scrum is accountable for the Sprint Backlog?

This technique is also known as swarming, and others have said, it is a positive. Limiting Work-in-progress (WIP) can be a good thing and may actually increase throughput and productivity. Check out Little's Law.