diff --git a/msx2utf8.py b/msx2utf8.py
index 8fee05e..90eeabd 100755
--- a/msx2utf8.py
+++ b/msx2utf8.py
@@ -27,10 +27,17 @@ def msxchars():
     for i in range(65,96):
         yield (256 + i, chars[160 + i])
 
-charmap = {}
+msx2uni = {}
+uni2msx = {}
 
 for m,c in msxchars():
-    charmap[m] = c
+    if m in msx2uni:
+        raise ValueError("MSX code {} is duplicated".format(m))
+    msx2uni[m] = c
+    if c in uni2msx:
+        raise ValueError("Codepoint {} ({}) is duplicated"
+                .format(ord(c), c))
+    uni2msx[c] = m
 
 if len(sys.argv) < 2:
     while True:
@@ -44,7 +51,7 @@ if len(sys.argv) < 2:
                 sys.stdout.write('\x01')
                 break
             code = 256 + c[0]
-        char = charmap[code]
+        char = msx2uni[code]
         sys.stdout.write(char)
 
 if sys.argv[1] == '-l':