Want to get list of ids containing duplicates

a!localVariables(
  local!list: {
    {id: 12, FacilityName: "New Test1", FacilityTypeId: 8},
    {id: 13, FacilityName: "New Test0", FacilityTypeId: 9},
    {id: 14, FacilityName: "New Test1", FacilityTypeId: 8},
    {id: 15, FacilityName: "New Test0", FacilityTypeId: 9}
  },

  local!withKey: a!forEach(
    items: local!list,
    expression: a!map(
      id: fv!item.id,
      key: fv!item.FacilityName & "-" & fv!item.FacilityTypeId
    )
  ),

  local!uniqueKeys: union(local!withKey.key, local!withKey.key),

  a!forEach(
    items: local!uniqueKeys,
    expression: a!localVariables(
      local!matchingIds: tointeger(local!withKey[wherecontains(fv!item, local!withKey.key)].id),
      if(count(local!matchingIds) > 1, local!matchingIds, {})
    )
  )
)

a!localVariables(
  local!list: {
    {id: 12, FacilityName: "New Test1", FacilityTypeId: 8},
    {id: 13, FacilityName: "New Test0", FacilityTypeId: 9},
    {id: 14, FacilityName: "New Test1", FacilityTypeId: 8},
    {id: 15, FacilityName: "New Test0", FacilityTypeId: 9}
  },

  local!withKey: a!forEach(
    items: local!list,
    expression: a!map(
      id: fv!item.id,
      key: fv!item.FacilityName & "-" & fv!item.FacilityTypeId
    )
  ),

  local!uniqueKeys: union(local!withKey.key, local!withKey.key),

  a!forEach(
    items: local!uniqueKeys,
    expression: a!localVariables(
      local!matchingIds: tointeger(local!withKey[wherecontains(fv!item, local!withKey.key)].id),
      if(count(local!matchingIds) > 1, local!matchingIds, {})
    )
  )
)

Sure, Thank so much giving me the implementation.  The result I got is {12, 14, 13, 15}. Is there a way I can get {  {12, 14},   {13,15}  }. Want to let the user know specifically that {12,14} and {13, 15} are duplicates?

Appian will always flatten nested lists.

Is there a way to not flatten the list in this case. Or its not possible

I think I found a way by using the toString function under for loop expression to return the list of strings as opposed to list of list. Thanks everyone!!