Original text:
The JSON value the JSONPath query is applied to is, by definition, a valid JSON value.  The parsing of a JSON text into a JSON value and what happens if a JSON text does not represent valid JSON are not <del> defined by this specification. When a JSONPath query is applied to a JSON value containing an object with duplicate names, the behaviour is unpredictable.  (Such a value usually originates in JSON text containing an object with duplicate keys, but how such a value could be constructed depends on how the JSON text is parsed and how the parsed value is represented.) </del>  3.2. 3.4.2.2. The "dot-selector" selects the node of the member value corresponding <del> to the member name from any JSON object in its input nodelist.  The behaviour is unpredictable if the JSON object has duplicate names. </del>  It selects no nodes from any other JSON value. A "dot-wild-selector" acts as a wildcard by selecting the nodes of all member values of an object in its input nodelist as well as all <del> element nodes of an array in its input nodelist.  The behaviour is unpredictable for an object with duplicate names. </del>  Applying the "dot-wild-selector" to a primitive JSON value (a number, a string, "true", "false", or "null") selects no node. The "index-selector" applied with a "quoted-member-name" to an object selects the node of the corresponding member value from it, if and <del> only if that object has a member with that name.  The behaviour is unpredictable if the object has duplicate names.  Nothing is selected </del>  from a value that is not a object. The "index-selector" applied with an "element-index" to an array 3.4.5.2. An "index-wild-selector" selects the nodes of all member values of an <del> object as well as of all elements of an array.  The behaviour is unpredictable for an object with duplicate names. </del>  Applying the "index-wild-selector" to a primitive JSON value (that is, a number, a string, "true", "false", or "null") selects no node. or <del> of their array elements or member values, respectively.  The behaviour is unpredictable for objects with duplicate names.  Applied to other value types, it will select nothing. </del>  A relative path, beginning with "@", refers to the current array element or member value as the filter selector iterates over the This definition does not stipulate the order in which the children of an object appear, since JSON objects are unordered. <del> The behaviour of a "descendant-selector" is unpredictable if the node or one of its descendants is an object with duplicate names. </del> The resultant nodelist of a "descendant-selector" applied to a node must be a sub-sequence of an array-sequenced preorder of the descendants of the node.

Reviewer comments:
Centralize the text that explains how non-interoperable JSON may cause unpredictable behavior. This is a little clumsy as the sections to be referenced are currently unnumbered.
Solves a few "behaviours", too :-)
This centralizing approach is a good thing. I suggest to reuse the phrase "duplicate names" for better recognition of the wording in RFC8259, though is more general, as it addresses more than two.
There may be scope for fine-tuning the section cross-referencing, but this can be deferred to purely editorial activity

Revised text:
The JSON value the JSONPath query is applied to is, by definition, a valid JSON value.  The parsing of a JSON text into a JSON value and what happens if a JSON text does not represent valid JSON are not  <ins> defined by this specification.  Sections RFC8259 and RFC8259 of RFC8259 identify specific situations that may conform to the grammar for JSON texts but are not interoperable uses of JSON, for instance as they may cause unpredictable behavior.  The present specification does not attempt to define predictable behavior for JSONPath queries in these situations.  (Note that another warning about interoperability, in RFC8259, at the time of writing is generally considered to be overtaken by events and causes no issues with the present specification.) Specifically, the "Semantics" subsections of Sections dot-selector, wildcard, index-selector, index-wildcard-selector, filter-selector, and descendant-selectors describe behavior that turns unpredictable when the JSON value for one of the objects under consideration was constructed out of JSON text that exhibits multiple members for a single object that share the same member name ("duplicate names", see RFC8259).  Also, matching (selectors) and comparing (comparisons in Section filter-selector) of strings assumes these strings are sequences of Unicode scalar values, turning unpredictable if they aren't (RFC8259). </ins> 3.2. 3.4.2.2. The "dot-selector" selects the node of the member value corresponding  <ins> to the member name from any JSON object in its input nodelist. </ins> It selects no nodes from any other JSON value. A "dot-wild-selector" acts as a wildcard by selecting the nodes of all member values of an object in its input nodelist as well as all  <ins> element nodes of an array in its input nodelist. </ins> Applying the "dot-wild-selector" to a primitive JSON value (a number, a string, "true", "false", or "null") selects no node. The "index-selector" applied with a "quoted-member-name" to an object selects the node of the corresponding member value from it, if and  <ins> only if that object has a member with that name.  Nothing is selected </ins> from a value that is not a object. The "index-selector" applied with an "element-index" to an array 3.4.5.2. An "index-wild-selector" selects the nodes of all member values of an  <ins> object as well as of all elements of an array. </ins> Applying the "index-wild-selector" to a primitive JSON value (that is, a number, a string, "true", "false", or "null") selects no node. or  <ins> of their array elements or member values, respectively.  Applied to other value types, it will select nothing. </ins> A relative path, beginning with "@", refers to the current array element or member value as the filter selector iterates over the This definition does not stipulate the order in which the children of an object appear, since JSON objects are unordered.  The resultant nodelist of a "descendant-selector" applied to a node must be a sub-sequence of an array-sequenced preorder of the descendants of the node.