Add check for writing to an already existing file

Author: MaggieKimani1

src/Microsoft.OpenApi.Tool/OpenApiService.cs

@@ -1,4 +1,4 @@
-using System;
+using System;
 using System.IO;
 using System.Linq;
 using System.Net;
@@ -63,6 +63,11 @@ public static void ProcessOpenApiDocument(
                 throw new ArgumentException(string.Join(Environment.NewLine, context.Errors.Select(e => e.Message).ToArray()));
             }
 
+            if (output.Exists)
+            {
+                throw new IOException("The file you're writing to already exists.Please input a new output path.");
+            }
+
             using var outputStream = output?.Create();
 
             var textWriter = outputStream != null ? new StreamWriter(outputStream) : Console.Out;