Forbid duplicate keys in strict mode (#121)

* Forbid duplicate keys in strict mode

Prevents specifying duplicate keys using
UnmarshallStrict: https://pkg.go.dev/gopkg.in/yaml.v2#UnmarshalStrict

* Add acceptance tests for duplicated properties

fixtures/duplicate_property.yaml

@@ -0,0 +1,18 @@
+---
+apiVersion: apps/v1
+kind: DaemonSet
+metadata:
+  name: nginx-ds
+spec:
+  replicas: 2
+  selector:
+    matchLabels:
+      k8s-app: nginx-ds
+  template:
+    spec:
+      containers:
+      - image: envoy
+        name: envoy
+      containers:
+      - image: nginx
+        name: nginx