Each Google Doc can only have one line that is in the "Title" style in Google Docs. You can see if a line is in the "Title" style by selecting it and looking in the box under "Format" in the menu, in a Google Doc, like in the screenshot below:

The top line of each document should be in the "Title" style. If you want other headings, lower down in the same document, they should be in the "Heading 1" style:

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