I am learning a song that has a V sus 4 chord and then in the guitar version of the same song the person who notated it used a Isus2 chord instead... I realize they sound very similar. Are they the same chord? Are there any other doubles of this nature in normal diatonic chords?
Here is a pedantic answer from a music theorist: They are both the same and different.
They are the same in terms of intervallic structure. They both contain a fourth, a fifth, and a second, which is a pitch class set referred to as [0,2,7] or Forte number 3-9. As such, they sound basically the same when heard in isolation; to me, they have a characteristic "open but pungent" quality. Another way to say this is that they are the same sonorism.
They are also the same in that they are both quartal chords (chords that can be formed by stacking fourths). This sort of harmony system was popular in the mid 20th century, the most famous example being the theme to Star Trek the Original Series.
That being said, they are also different in terms of functional harmony (a.k.a. Beethovian-tradition western tonalism, which is the basis of most popular music today). In the Vsus4 chord, the G and C form a dissonant fourth, while in the Isus2 chord, they are an inversion of a perfect fifth (see this link for the reason why). Therefore they have different tendencies and serve a different functional role in tonal music. The Vsus4 is a dominant chord, where the C will tend to want to resolve downward, while the Isus2 is a tonic chord that does not require resolution.
If you are analyzing music, you should consider the context when deciding whether a chord of this type is a Isus2 or a Vsus4. For most applications, these chords are considered functionally different.
So I've learnt something again: The 3rd is missing in a sus2!
(like piperii says, the root will be defferent: I = tonic V = dominant)
Here are some good pictures explaining the situation:
Isus2 in key C consists of C D and G. The 3rd (E) is lost in favour of the 2nd (D).
Vsus4 in key C consists of G C and D. The 3rd (B) is lost in favour of the 4th (C).
Thus both have exactly the same notes. So it could be conceded that they are the same chord, albeit with different voicings. However, a I chord is root based, while a V chord is dominant. So they have different functions. Partially, it's up to the writer to decide which will be the 'correct' one in its position in a piece, so 'the writer' has the choice of name.
Usually, the root note will be the decider, so here, if C is better underneath, it'll be Csus2, otherwise vice versa.