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When typewriters were used to type essays there was a need for two spaces after the period. The font type was uniformly spaced so that the letter “i” took up the same amount of space as the letter “b.” Two spaces at the end of the sentence visually broke up the type. With modern type font on a computer where each letter only takes up as much space as it needs, the need for the extra space is not necessary.

Habits Die Hard

If you are in the habit of two spaces after the period you are likely to make this error sometimes. Even without being in the habit of two spaces, occasionally we will hit an extra space between words.

Control F

Control F on a PC (Command F on a Mac) does a FIND. Instead of typing a word into the find box, type two spaces.

Dot Dot Dot

If you are using Google Docs you will notice that there is a button with a dot dot dot icon. This allows you to open a dialogue box to replace. 
google find and replace

Replace

In the find and replace, find two spaces and replace with one space. Replace all. I try to always do this before I submit a paper. Usually there is at least one place that I made this mistake.
Replace two spaces with one space