For a node using \relates QDoc writes it twice to the index; once under the original (lexical) parent (typically, under the global scope), and again under the adoptive parent specified with \relates. This is by design, as it allows searching for the node under both scopes.
However, when the nodes are read back in, what was originally a single node is now duplicated. This causes issues e.g. when that node is part of a group; when generating a list for a group, a line representing the node appears twice.