Trying to find duplicate rows and then delete them (also type mismatch error)

Question

I have rows of data from B18 until whatever, there is always data being added on and removed. I want to implement an error check that checks for duplicate rows, until it reaches empty cells. Then I want to shift all the data up so that there isn't just a blank row.

Right now (to start of small) I am just trying to highlight the duplicate data, and I tried a few different things, for example I found a similar code for this on here, but I am getting a type mismatch error. Any help would be appreciated :)

Dim firstRow As Long
Dim lastRow As Long

    firstRow = 19
    lastRow = 500
        
    For j = firstRow To lastRow
        If Range("B" & j & ":H" & j).Interior.Color = RGB(255, 0, 0) Then
    Else
        For i = j + 1 To lastRow
            If Range("B" & i & ":H" & i).Interior.Color = RGB(255, 0, 0) Then
        ElseIf Range("B" & i & ":H" & i).Value = Range("B" & j & ":H" & j).Value Then
            Range("B" & i & ":H" & i).Interior.Color = RGB(255, 0, 0)
            Range("B" & j & ":H" & j).Interior.Color = RGB(255, 0, 0)
        End If
        Next i
    End If
    Next j

Update:

I also tried the Worksheet function, and I thought of having nested if loops and comparing each column individually if the previous one matched. However this didn't work as it would compare with all the cells in the column instead of the ones that have cells matching in another column. Ignore the fact that cell C is actually F, I was testing something.

            Dim lastRow As Long
        Dim rngB As Range
        Dim rngC As Range
        Dim cB As Range
        Dim cC As Range
        
        lastRow = existingSheet.Cells(existingSheet.Rows.Count, "B").End(xlUp).Row + 1
        
        Set rngB = existingSheet.Range("B19:B" & lastRow)
        Set rngC = existingSheet.Range("F19:F" & lastRow)
        
        For Each cB In rngB
            If Application.WorksheetFunction.CountIf(rngB, cB.Value) > 1 Then
                Set cC = existingSheet.Cells(cB.Row, "F")
                
                If Application.WorksheetFunction.CountIf(rngC, cC.Value) > 1 Then
                    cB.Interior.Color = RGB(255, 0, 0) ' Red color
                    cC.Interior.Color = RGB(255, 0, 0)
                End If
            End If
        Next cB