According to Unihan database some Chinese characters have multiple simplified variants.
For example the Chinese character 鍾 have 2 simplified variants: 钟 and 锺
How it could happen? What is the origin of such situation?
It is almost the case that each Traditional character maps to exactly one Simplified character (possibly itself). This is certainly the mental model that most people have about simplification, and it's not far from the truth.
One of my favorite references on this topic lists out ambiguities in simplification. The "2 to 1" cases at the bottom covers the cases when a single traditional character has multiple simplified characters (Also check out the crazy special case!). In all the 2-to-1 cases, it seems that a semantic distinction (possibly also present in pronunciation) is reflected in the written form of the simplified (but not the traditional). Examples:
征(zhēng)(archaic name for a pentatonic tone vs to conquer)
划(to draw vs. to plan)
Two of the four entries in the hanzidb list you provided are accounted for by that Wiki page (as well as several others that the hanzidb list left out). I don't know how the list was created, so I'm speculating a bit more in addressing the others:
锺 - 曾作“鍾”的简化字，后停用。Previously served as the simplification for
鍾, but it is no longer used. Unrelated to this specific character, there are actually a whole bunch of simplified variants that resulted from a failed 2nd round of simplification. Some of these are really crazy (but most aren't in unicode).
願 -> 愿𫖸(the second one doesn't render for shit, so here's an image
Also, another note. There are also what Unihan refers to as "z-variants", meaning typographic variants of characters. Both the Wiki page and the hanzidb list have (correctly) left these out. They aren't very interesting, but sometimes cause issues when you're doing data processing of CJK text:
Sorry, but the simplified words actually are different.(Please don't argue, I'm Chinese and speak Mandarin fluently.) Their Pin Yin might be spelled the same, but they mean different things and are pronounced differently.
For those of you who don't know what Pin Yin is, it's basically a spelling of the pronunciation of a character in English letters and four different accents to represent "tones" of a word..
To read more about Pin Yin, go on this website: https://en.wikipedia.org/wiki/Pinyin
I'm sorry it's Wiki, but I couldn't find anything better than that. So not everything on that website is accurate.