This question is quite similar to Tuning of Accidentals & Scales, and you may find some useful information there.
Some people say C# and Db are same, but some say they are not.
The most precise way to say it is that they are 'enharmonic equivalents'. Whether you think they are the same 'note' is a matter of opinion, and depends on your definition of 'note'.
Often they are the same pitch; sometimes they aren't.
Some say, they both have same frequency in 12 tone system.
As you mention in your question, just knowing the note name doesn't define a frequency (or pitch). You need to know the note name, the reference concert pitch, and the temperament and intonation to which the instrument is tuned (for a fixed pitch instrument like a piano), or with which it is being played (for a continuous pitch instrument like a violin).
Is there any other music system where those have different frequency?
Simplistically speaking, you can think of an infinite number of combinations of situations where a C# played with one combination of concert pitch, tuning and temperament would have a different pitch to a Db played with another.
One a single given instrument on any given day, they might have a different frequency any time a player can play them differently, and chooses to.
I also heard that C# is actually have less frequency (Hz) than Db
That may be true in some situations, but it's not true as a general rule. As above, you need to define more than just the note name to be able to calculate the frequency of a note.
I saw that C# Major Scale and Db Major scale have same notes in it. So, can we say both are same. ?
It's the same as my answers above for a single note. Are you talking about an instrument where the player can't play the enharmonically equivalent notes differently? If so, they will have the same frequency. If not, then players may choose to play them differently.
Generally, singers and players of continuously-pitched instruments tend (consciously or unconsciously) towards using just intonation in the key that they're playing in. In those cases, it might be very common for a given note that we're calling C# to have a different frequency to one we're calling Db.
I heard that there exist piano for separate keys for sharps and flats ?
I think there are some historic instruments that do - I'm not personally an expert, but hopefully someone will be. It's not commonly-found in modern instruments though.