Duplicate “dt” name “X” in “dl” element. Within a single “dl” element, there should not be more than one “dt” element for each name.

Within a <dl>, each name should appear in only one <dt>, so a term with several values gets one <dt> followed by several <dd> elements.

A description list is a set of name-value groups, and the HTML specification says a single <dl> should not have more than one <dt> for each name. When the same term shows up twice with different values, readers can't tell whether the second entry replaces the first, adds to it, or is a mistake. Repeated terms often come from lists built from data, such as product specifications where one attribute is output twice.

The validator compares the text content of each <dt>. It ignores markup and leading or trailing whitespace, so <dt><b>Size</b></dt> and <dt> Size </dt> both count as Size. The comparison is case-sensitive, so Size and size are different names. Wrapping each group in a <div> doesn't help, because the check covers the whole <dl>. Along with this warning, the validator adds a second message pointing to the first <dt> with that name.

If both values belong to the same term, keep one <dt> and put each value in its own <dd>. If the entries describe different things that share a label, give each <dt> a distinct name, or move them into separate <dl> elements, since the rule applies only within a single list.

Invalid example

<dl>
  <dt>Size</dt>
  <dd>Small</dd>
  <dt>Color</dt>
  <dd>Red</dd>
  <dt>Size</dt>
  <dd>Medium</dd>
</dl>

Valid example

<dl>
  <dt>Size</dt>
  <dd>Small</dd>
  <dd>Medium</dd>
  <dt>Color</dt>
  <dd>Red</dd>
</dl>

Valid example with distinct names

<dl>
  <dt>Net weight</dt>
  <dd>1.2 kg</dd>
  <dt>Gross weight</dt>
  <dd>1.4 kg</dd>
</dl>

Learn more:

MDN Reference

MDN Web Docs: The Description List element

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MDN Reference

MDN Web Docs: The Description Term element

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WHATWG Spec

HTML Standard: Grouping content

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