[Rob557]

The Aug, 24 2018 Calibre release 3.30 has the following new feature:

"ToC Editor: When generating ToCs using headings/XPath ignore duplicate entries at the same level that have the same text."

This (new) feature is currently non-optional and one circumstance where it causes a problem is with ePubs that have multiple parts and each part re-starts the chapter numbering. If the first part has five chapters (#1-5) and the second has seven (#1-7), then the first five chapters in the second part are excluded when trying to build the ToC.

A ToC generator that focuses only on the chapter numbers will display chapters 1-7 with no indication that there are two parts and that five of the twelve chapters are not displayed.
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ERROR - the above comments refer to the new feature as "non-optional" but as the Ducks points out in later comments below, there is an easy option to turn off the feature.