When double o occurs, the second /o/ syllable is written with hiragana う, instead of お. Thus, for instance, the word ohayoo (good morning) is written as おはよう, not as おはよお.
There are a few exceptions to this rule. For instance, the word ookii (big) is written as おおきい, instead of おうきい.
When double e occurs, the second /e/ syllable is written with hiragana い, instead of え. For instance, the phrase situreesimasu (excuse me) is written as しつれいします instead of しつれえします.
Again, there are a few exceptions to this rule. For instance, the word oneesan (older sister) is written as おねえさん, instead of おねいさん.
Although exceptional cases (for both double o's and double e's are very few, it is always good to check with your Japanese version of JSL for transcribing those double vowels.
Other double vowels are simply written as they are pronounced. Examine, for instance, the following:
おかあさん okaasan (mother)
おいしい oisii (delicious)
You should also be aware that there are many word pairs which look alike except for a repeated vowel in one of them that makes a difference in the meaning. Here are a few examples:
おばさん obasan (aunt) vs. おばあさん obaasan (grandmother)
おじさん ozisan (uncle) vs. おじいさん oziisan (grandfather)
にんぎょ ningyo (mermaid) vs. にんぎょう ningyoo (doll)
Whenever double consonants occur (i.e. pp, tt, kk, ss) the first of the pair is always written with a "half size" つ which looks like this: っ. Observe the following examples:
がっこう gakkoo (school)
にっぽん nippon (Japan)
ざっし zassi (magazine)
きって kitte (postal stamp)