While watching many movies, I begin noticing that several titles have very similar or even identical storyline except that the names of the title, characters, places, etc. are different.

Usually, they are in pair. One came from independent or less popular film, and one came from major film (eg. Paramount, Universal, etc.) and featured top actors/actress. The first one was always available prior the latter one.

There are three pairs that I noticed.
The newset pair is "Transformer" and "Transmorpher".
Then "Davinci Code" and "Davinci Treasure".
Also "Snow Dogs" (Walt Disney) and "Kevin of the North".

So, my question is, how can this be possible?

It's a very good question (and an excellent observation). The answer is complicated, though.

Let's look at a few things:

Not only are films allowed to have similar titles, they are even allowed to have the SAME title and be a completely different film. For example, there are three different films called "The Day After Tomorrow" but they are about completely different subjects. Sometimes there is a title which may have been used for a dozen different films.

This is allowed as long as it's not a very recognizable name. For example, you can't make a film and call it "Casablanca" or "The Godfather," since those names are well-recognized.

Then there are remakes, which are films which have the same title and are basically the SAME film, just a newer version of it. That happens a lot. For example, "The War of the Worlds" was made in 1953 and was remade in 2005. It's basically the same story since it was based on the same book.

Then you have different versions of the SAME story with DIFFERENT names. For example, many films have been made about the sinking of the Titanic. One was called "A Night to Remember" and was made in 1958. It was actually originally titled "Titanic." The recent one is called "Titanic" and was made in 1997. BUT there were FIVE other films titled "Titanic," and they were all about the same subject.

So you are allowed to have different films about different subjects with the same title, different films about the SAME subject with the SAME title, and different films about the SAME subject with DIFFERENT titles.

All are allowed and occur very often.

Hmm...
Based on your explanation, all the title pairs in my list fall into the SAME subject, DIFFERENT title category.

But the last pair ("Snow Dogs" and "Kevin of the North") has identical storyline and not just the same subject. About 90% of the scenes are identical. It was like two different producers trying to create a movie based on the exact same book.

I don't know what was going on behind the making of those two film, but unless the author of the book (or the publisher) actually gave permission/license to both of the producers, shouldn't this be a copyright issue?

That's true, but they aren't necessarily two movies based on the same exact book, especially if it's a book based on a true story.

Different producers can make films based on the same story at the same time. It's actually happening right now, with two films based on the life of San Francisco supervisor Harvey Milk who was assassinated. But they aren't necessarily based on the same book. While an author or publisher will option a book to a producer to be made into a film, there is nothing to prevent another producer from making a film about the same subject. This is especially easy when it is based on a true story, like the Iditarod sled dog race.

Two movies cannot be made about the same book of fiction since the author can only sell the rights one time. But if a book is written about a true story, anybody can write a book about that same story and any producer can make a film about it.