When Greek spelling was reformed in the 1980s, every accent was dropped except the oxia (acute). But experiments in typography at that time (e.g., rendering the acute nearly erect, or in the shape of a triangle), combined with the orthographical reforms, gave the false impression that all previous accents had been discarded in favor of a new and different accent, the tonos. Hence double code points for the vowels with oxiai made their way into fonts and into pre-Unicode encodings that were eventually folded into Unicode (which had to deal with legacy code tables). This legacy is evident in Unicode, which designated two major blocks:
The Unicode Consortium soon recognized that the duplicated vowels preserved a false distinction. They therefore established normalizing rules that dictated that the upper code point should always be converted to the lower. But it is common to run into the problem of acute vowels encoded in Greek Extended.
For some reason, perhaps because of an oversight, or perhaps because the editors thought that there was some essential difference between tonos and oxia (acute), which there is not. Sixteen characters in the Greek basic set were duplicated in Greek extended:
|Unicode||Beta Code||Basic codepoint||extended codepoint|
There is no semantic difference between, for example, ά and ά (both alpha-oxia), so in most cases you don't need to worry about this. Your Greek Keyboards (Unicode) will make a decision and input one or the other. A search engine should be able to find both from either input (just as they should be able to strip diacritics altogether from a search term, if desired).
The Unicode database dictates a rule for normalization that converts the higher code point to its corresponding lower code point.
Most Greek Fonts (Unicode) will display both versions identically. A notable exception is Adobe Garamond Premiere Pro, which places the accent in the lower code point versions at almost a vertical angle, and the upper code point versions at a slanted, ca. 45-degree angle. This can cause a problem with software that (correctly) normalizes the higher points to the lower ones, producing an inconsistent appearance of accentuation.
There have been problems with this (certain fonts in certain browsers don't get it right, for example). It seems (see discussion from GreekKeys Unicode) that the higher codepoint, in Extended Greek, has been de facto deprecated, by virtue of Unicode normalization rules that turn the higher code point vowels into their lower code point equivalents. By preference all tools and input methods should use the Basic Greek vowel+tonos for these character combinations. Fonts and search tools should continue to support both for the sake of legacy data.