Return error if there are multiple same name params

Right now pipeline accept multiple parameters with
the same name. This sometime causes the pipeline to
not finish.

This change adds a simple check and returns error
when there are multiple parameters with the same
name in the spec.params.

pkg/apis/pipeline/v1beta1/pipeline_validation.go

@@ -341,6 +341,9 @@ func validatePipelineParameterVariables(tasks []PipelineTask, params []ParamSpec
 			}
 		}
 
+		if _, ok := parameterNames[p.Name]; ok {
+			return apis.ErrGeneric("parameter appears more than once", fmt.Sprintf("spec.params.%s", p.Name))
+		}
 		// Add parameter name to parameterNames, and to arrayParameterNames if type is array.
 		parameterNames[p.Name] = struct{}{}
 		if p.Type == ParamTypeArray {

pkg/apis/pipeline/v1beta1/pipeline_validation_test.go

@@ -916,6 +916,39 @@ func TestValidatePipelineParameterVariables_Failure(t *testing.T) {
 				Name: "a-param", Value: ArrayOrString{Type: ParamTypeArray, ArrayVal: []string{"value: $(params.baz[*])", "last"}},
 			}},
 		}},
+	}, {
+		name: "multiple string parameters with the same name",
+		params: []ParamSpec{{
+			Name: "baz", Type: ParamTypeString,
+		}, {
+			Name: "baz", Type: ParamTypeString,
+		}},
+		tasks: []PipelineTask{{
+			Name:    "foo",
+			TaskRef: &TaskRef{Name: "foo-task"},
+		}},
+	}, {
+		name: "multiple array parameters with the same name",
+		params: []ParamSpec{{
+			Name: "baz", Type: ParamTypeArray,
+		}, {
+			Name: "baz", Type: ParamTypeArray,
+		}},
+		tasks: []PipelineTask{{
+			Name:    "foo",
+			TaskRef: &TaskRef{Name: "foo-task"},
+		}},
+	}, {
+		name: "multiple different type parameters with the same name",
+		params: []ParamSpec{{
+			Name: "baz", Type: ParamTypeArray,
+		}, {
+			Name: "baz", Type: ParamTypeString,
+		}},
+		tasks: []PipelineTask{{
+			Name:    "foo",
+			TaskRef: &TaskRef{Name: "foo-task"},
+		}},
 	}}
 	for _, tt := range tests {
 		t.Run(tt.name, func(t *testing.T) {