Duplicate in the grid

What would "duplicate values" look like in this case?  Are you talking about a grid of data being created / populated by the user at run-time, or something else?

In the grid, for a particular field I select values from another table. The selected value appears in the grid. 
Now, I am able to select the same value twice. But, it should not happen.

For example, if I have the same value twice, it should throw the error, "duplicate values are present". please check

You can add a validation item on your grid, which checks whether there are duplicate values within that column.  I wrote a rule a while ago which looks into an array and returns any items that are duplicated within the array any number of times, which I'll share below.

For the validation message on your grid:

a!validationMessage(
  message: "Duplicate Amounts are present.",
  validateAfter: "SUBMIT",
  showWhen: length(
    rule!GLBL_getArrayDuplicates(local!gridData.Amount)
  ) > 0
)

For the rule GLBL_getArrayDuplicates():

a!flatten(
  a!forEach(
    ri!array,
    if(
      length(wherecontains(
        fv!item,
        cast(typeof({fv!item}), ri!array)  /* (this allows us to handle a mixed-type array properly) */
      )) > 1,
      fv!item,
      {}
    )
  )
)

You can add a validation item on your grid, which checks whether there are duplicate values within that column.  I wrote a rule a while ago which looks into an array and returns any items that are duplicated within the array any number of times, which I'll share below.

For the validation message on your grid:

a!validationMessage(
  message: "Duplicate Amounts are present.",
  validateAfter: "SUBMIT",
  showWhen: length(
    rule!GLBL_getArrayDuplicates(local!gridData.Amount)
  ) > 0
)

For the rule GLBL_getArrayDuplicates():

a!flatten(
  a!forEach(
    ri!array,
    if(
      length(wherecontains(
        fv!item,
        cast(typeof({fv!item}), ri!array)  /* (this allows us to handle a mixed-type array properly) */
      )) > 1,
      fv!item,
      {}
    )
  )
)