you can merge them if you are 100% sure they are the same person
The other reply is correct for deceased people, if they are the same then merge but remember living are treated different. Because of privacy living each have their own ID, for example my brother and I both have a different ID for my living mother that connects us to the tree. We cannot merge mother's living IDs, you cannot merge living person so there will be multiple copies of living until they pass away, then we will merge. (just did this for my father who passed away last year and there were copies)
Some times there are duplicates where one is marked as living and one is marked as deceased
in which case you need to mark as deceased the one that is marked as living (assuming the person is deceased)
Once both people are shown as deceased you can merge the two into one. Just be careful they are the same person. Looking at the source documents will help in determining this.
You can merge two living individuals as long as you can see both of them. Copy the ID for one, then go to the other and click Merge by ID in the Tools box on the right. Paste the ID into the box and then you can merge the two.