Rules can involve several copies of the same relation, corresponding to multiedges in a graph. A simple example is the rule:
{{x, y}} -> {{x, z}, {x, z}, {y, z}}
RulePlot[ResourceFunction[
"WolframModel"][{{x, y}} -> {{x, z}, {x, z}, {y, z}}],
VertexLabels -> Automatic, "RulePartsAspectRatio" -> 0.25]
Running this rule produces a structure with 3n edges and nodes at step n:
ResourceFunction[
"WolframModel"][{{{x, y}} -> {{x, z}, {x, z}, {y, z}}}, {{1, 1}},
5]["StatesPlotsList", "MaxImageSize" -> 180]
Rules can both create and destroy multiedges. The rule
{{x, y}} -> {{x, z}, {z, w}, {y, z}}
RulePlot[ResourceFunction[
"WolframModel"][{{x, y}} -> {{x, z}, {y, z}, {z, w}}],
VertexLabels -> Automatic, "RulePartsAspectRatio" -> 1]
generates a multiedge after one step, but then destroys it:
ResourceFunction[
"WolframModel"][{{{x, y}} -> {{x, z}, {z, w}, {y, z}}}, {{1, 1}},
5]["StatesPlotsList", "MaxImageSize" -> 180]