drop banner in text part too if found and insert at end

mime-inject.py

@@ -48,8 +48,11 @@ def inject(m):
     e = m.get('Content-Transfer-Encoding','').lower()
     payload = m.get_payload(decode=True).decode(m.get_content_charset("utf-8"))
 
-    if c == 'text/plain' and txt not in payload:
-        payload += f"\n{txt}\n"
+    if c == 'text/plain':
+        if txt.strip() in payload:
+           payload = payload.replace(txt.strip(), '')
+
+        payload = f"{payload.strip()}\n\n{txt}\n"
 
     elif c == 'text/html':
         h = HTML.fromstring(payload)