How do I undo this splitting of text into columns?

I've received an ODT file, where the text, intended to have usual page-wide layout appears to be split in two columns. If I copy the text and paste as plain text, I get this:

1
2
3
4
5
6
7
8

But in LibreOffice Writer it looks instead like this:

I've tried looking into Format->Page->Columns, but there I have Columns spinbox read 1, so there the setting is single-column layout. Apparently, there's more than one way to have two columns.

What is the feature this document is using? How do I tu it off so that the document became single-column?

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