Ryakuji

Ryakuji (kanji:略字, kana:りゃくじ) are shorthand and usually unofficial forms of kanji. 

For example, 々 is a shorthand form of the kanji 仝, which itself is another form of the kanji 同, which means "same". For compound words where the same kanji is used twice in a row, 々 is used where the second kanji would be so that it doesn't have to be written again. 

For example, the word tokidoki, which means "from time to time", uses the same kanji for "time" twice, but is written as 時々 instead of 時時 because Japanese would see the second compound as redundant. So 々 implies that the second character is the same as the first.

Other same character compounds include:
+KanjiKanaRōmajiMeaning色々いろいろiro-irovarious神々かみがみkami-gamithe gods島々しまじまshima-jimaislands
This is similar to English ligatures, or letters that combine to make a new letter or symbol such as &, which was a combination of the letters that make the Latin word "et".

 References 

Category:Japanese writing system