if duplicate data exists in the list then display a message indicating that duplicate data is present.

You can build an expression rule to check duplicates and call that in the validations section of the grid/section. 

a!localVariables(
  local!data: {
    a!map(SAPNumber: "1111111", Qty: "1", Spare: "0"),
    a!map(SAPNumber: "1111111", Qty: "2", Spare: "1"),
    a!map(SAPNumber: "1111111", Qty: "3", Spare: "0"),
    a!map(SAPNumber: "2222222", Qty: "1", Spare: "0")
  },
  if(
    contains(
      a!foreach(
        local!data,
        a!localVariables(
          local!arrayTemp: remove(local!data, fv!index),
          local!SAPCheck: property(
            local!arrayTemp,
            wherecontains(
              fv!item.SAPNumber,
              local!arrayTemp.SAPNumber
            ),
            {}
          ),
          local!spareCheck: property(
            local!arrayTemp,
            wherecontains(fv!item.Spare, local!arrayTemp.Spare),
            {}
          ),
          a!isNullOrEmpty(
            intersection(local!SAPCheck, local!spareCheck)
          ),
          
        )
      ),
      true()
    ),
    "Validation Message",
    {}
  )
)

a!localVariables(
  local!data: {
    a!map(SAPNumber: "1111111", Qty: "1", Spare: "0"), 
    a!map(SAPNumber: "1111111", Qty: "2", Spare: "1"),
    a!map(SAPNumber: "1111111", Qty: "3", Spare: "0"),
    a!map(SAPNumber: "2222222", Qty: "1", Spare: "0")
  },
  
 a!forEach(
    items:local!data,
    expression:a!localVariables(
      local!innerData:index(local!data,fv!index,null()),
      local!SAPnumber:property(local!innerData,"SAPnumber"),
      local!spare:property(local!innerData,"Spare"),
      if(
        contains(
          property(remove(local!data,fv!index),"SAPnumber"),
          local!SAPnumber
        ),
        if(
          contains(
            property(remove(local!data,fv!index),"Spare"),
            local!spare
          ),
          "Duplicates Found",
          null()
        ),
        null()
       )
      )
    )
 )

a!localVariables(
  local!data: {
    a!map(SAPNumber: "1111111", Qty: "1", Spare: "0"),
    a!map(SAPNumber: "1111111", Qty: "2", Spare: "1"),
    a!map(SAPNumber: "1111111", Qty: "3", Spare: "2"),
    a!map(SAPNumber: "2222222", Qty: "1", Spare: "0")
  },
  local!dataset: a!foreach(
    local!data,
    a!localVariables(
      local!arrayTemp: remove(local!data, fv!index),
      local!SAPCheck: property(
        local!arrayTemp,
        wherecontains(
          fv!item.SAPNumber,
          local!arrayTemp.SAPNumber
        ),
        {}
      ),
      local!spareCheck: property(
        local!arrayTemp,
        wherecontains(fv!item.Spare, local!arrayTemp.Spare),
        {}
      ),
     
        a!isNotNullOrEmpty(
          intersection(local!SAPCheck, local!spareCheck)
        )

    )
  ),
  if(
    contains(local!dataset, true()),
    "Validation Message",
    ""
  )
)

 Rishu Kumar Gupta I noticed an issue in the above code while testing with a negative dataset, here is an adjusted one to resolve that. Pls refer this one. 

Thankyou Harsha Sharma 

Thankyou PavanSrihari 

A simple approach is to create a list of combined keys, get the unique combined keys, and then compare the number of combined keys to the number of items in your list. Below the code:

a!localVariables(
  local!data: {
    a!map(SAPNumber: "1111111", Qty: "1", Spare: "0"),
    a!map(SAPNumber: "1111111", Qty: "2", Spare: "1"),
    a!map(SAPNumber: "1111111", Qty: "3", Spare: "0"),
    a!map(SAPNumber: "2222222", Qty: "1", Spare: "0")
  },
  local!combinedKey: a!forEach(
    items: local!data,
    expression: concat(fv!item.SAPNumber, "-", fv!item.Spare)
  ),
  if(
    count(local!data) <> count(union(local!combinedKey, local!combinedKey)),
    "Validation Message",
    ""
  )
)

Another simple way to do this is as below, Note: I have not handle any null errors etc.. just a targeted answer for the requirement

a!localVariables(
  local!data: {
    a!map(SAPNumber: "1111111", Qty: "1", Spare: "0"),
    a!map(SAPNumber: "1111111", Qty: "2", Spare: "1"),
    a!map(SAPNumber: "1111111", Qty: "3", Spare: "0"),
    a!map(SAPNumber: "2222222", Qty: "1", Spare: "0")
  },
  if(
    length(local!data) > length(
    /* This union can be a rule named like XXX_removeDuplicate() */
      union(
        merge(local!data.SAPNumber, local!data.Spare),
        merge(local!data.SAPNumber, local!data.Spare)
      )
    ),
    "Error Message",
    ""
  )
)