Dataset.techniques field could have more than one items with duplicated pid

Issue Name

Dataset.techniques field could have more than one items with duplicated pid

Summary

When adding a new dataset it is possible to add an array of techniques with the same pid. This is possible because mongodb/mongoose only ensures that the document is unique but not ensures that all the document inside the array has unique pid.

Steps to Reproduce

• Send POST request to /datasets with

techniques : [
    {
      "pid": "2",
      "name": "a"
    },
    {
      "pid": "2",
      "name": "a"
    }
  ]

• Send a GET request to get your newly added dataset or check out this dataset from the database you will see the technique array with two elements which has the same pid.

Current Behaviour

Is able to have two or more techniques with the same pid in the array.

Expected Behaviour

Should not be able to add dataset with duplicated pids in technique array. Should return 422.

Start at the POST /datasets endpoint and the validation or schema handling for the techniques array, then reproduce the request shown in the issue. Done means datasets containing duplicate technique pids are rejected with a 422 response while valid arrays remain accepted.