Duplicate characters in Unicode

Unicode has a certain amount of duplication due to aiming to allow legacy encodings to be converted to Unicode without losing any information. Sometimes these characters are rendered identically, other times they are rendered with different sizes or styles at least in the fonts intended to match up with the expectations of legacy systems.

CJK fullwidth forms

In traditional CJK encodings characters usually took either a single byte (known as halfwidth) or two bytes (known as fullwidth). Characters that took a single byte were generally displayed at half the width of those that took two bytes. Some characters such as the latin alphabet were available in both halfwidth and fullwidth versions. As the halfwidth versions were more commonly used they were generally the ones mapped to the standard code points for those characters. Therefore a separate section was needed for the fullwidth forms to preserve the distinction.

Greek

Many Greek letters are used as technical symbols. All of the Greek letters are encoded in the Greek section of Unicode but many are encoded a second time under the name of the technical symbol they represent. Of these, micro sign is in the Latin-1 range and most of the rest are in the Letterlike Symbols range. The "micro sign" (U+00B5, µ) is obviously inherited from ISO 8859-1, but the origin of the others is less clear.

Technically these are not duplicate charactrs in that the consortium viewed these symbols used in mathematics as distinct charcters from their twins in hte Greek alphabet. For example, the upper-case charcter pi, Π typically shares the same glyph with the mathematical character n-ary product, ∏. These are two different characters because they each represent different semantics — different meanings. They nevertheless share similar presentational form: the glyph. Humans can typically percieve the difference in characters through the context in which the glyph appears. On the other hand, it is difficult to develop software that could disinguish these different meanings through context: hence the justification for pvoiding different charcters for these different meanings.

Roman numerals

Unicode has a number of characters specifically designated as Roman numerals, as part of the Number Forms range from U+2160 to U+2183. For example, MCMLXXXVIII could alternatively be written as ⅯⅭⅯⅬⅩⅩⅩⅧ. This range includes both upper- and lowercase numerals, as well as pre-combined glyphs for numbers up to 12 (Ⅻ or XII), mainly intended for the clock faces for compatibility with non–West-European encodings. The pre-combined glyphs should only be used to represent the individual numbers where the use of individual glyphs is not wanted, and not to replace compounded numbers. Similarly precombined glyphs for 5000 and 10000 exist.