A family of Microsoft word processing software products for creating web, email, and print documents.
It appears that when you press Enter, you are getting a paragraph with an indent or a first-line indent. As Stefan suggests, this may be because the "Style for following paragraph" is different from the style of the paragraph at the end of which you're pressing Enter. For example, the following style for all the built-in heading styles is Normal. If you have modified the Normal style to have some sort of indent, that could be the issue. But since you say pressing Enter twice solves the problem, the likelihood is that your paragraph is in the List Paragraph style because it is numbered or bulleted or in some way part of a list.