Translation:I am just looking.
In Chinese you don't use an explicit continuous marker to say this. If you did it would sound a bit like "I'm in the process of looking" in English, grammatical but odd.
But beyond that I can't decode the whole sentence. It would be good to hear from a Chinese native speaker.
It is a particle used for the present continuous.
There's nothing about languages that says there is only ever allowed to be one particle for one job. If "will" is the particle used for the future tense in English, then what's "shall"? And "going to" and "gonna"?
Chinese is no different. One word or particle can have one or more than one meaning or function. One grammatical function can be achieved by one or more than one word or particle.
在 and 着 are the two main particles for the continuous. Note also that it's not compulsory to indicate the continuous at all in Chinese and it might only be used for emphasis. This article might interest you: https://resources.allsetlearning.com/chinese/grammar/Expressing_actions_in_progress_(full_form)
是 means "is", "are", "be".
Native speaker here (long time expat though): I would use 在(doing) or more explicitly 正在. 在 also has the meaning of to be when referring to locations and location-related metaphors, while 是 in this use case would be unusual for me because it is more used to refer to being certain things. ie. I am a doctor 我是医生, I am superman: 我是超人. There might be regional differences idk about though
My chinese teacher told us that repeating a verb is like it will take a short time: 看看 would mean something like "take a look for a second!". Let me check if I find something out there. Ah, here it is: https://resources.allsetlearning.com/chinese/grammar/Reduplication_of_verbs
Chinese doesn't have past tense and as such 了 is never mandatory and has multiple uses. English "just" also has multiple uses including immediate past and "merely, only".
But since my earlier comment it now strikes me that the "merely, only" sense is clearly intended. But my Chinese isn't good enough so I'd still like to hear from a native speaker (-:
It's not even that, though redundant language would be bad, "only just" has a different meaning to only or just by themselves. "Only just" means barely or for only a short time as in 刚. E.g. "I only just caught the bus, a second later and I would have missed it." or "I only just started learning Chinese, so I can't read this yet."
I think the main translation for this sentence is wrong therefore because it sounds more like "I only just (started) looking" not "I am just browsing the shop" which is the true meaning.
Use only OR just, but not both.
"I am only just having a look" is a common English phrase.
In English you need to put the "am" which is the 是 in the Chinese sentence . But you are correct, you have understood, it just won't take it.
The Chinese 我只是随便看看 literally means:
"I only am pleasing myself looking", or "I only am -as I wish - looking".
In many ways I wish Duolingo would use direct translations like this instead of trying to make them perfect from an English point of view. The benefit would be that you would remember the Chinese word order. I think that would be far more useful even if it sounds funny in English.