diff --git a/lib/bb/utils.py b/lib/bb/utils.py
index 82dab6b..fa5d2ab 100644
--- a/lib/bb/utils.py
+++ b/lib/bb/utils.py
@@ -159,6 +159,8 @@ def explode_dep_versions(s):
             lastver = lastver + " " + (i[:-1] or "")
             r[lastdep] = lastver
         elif not inversion:
+            if i in r:
+                raise ValueError("Error, item %s appeared in dependency string '%s' multiple times.  explode_dep_versions cannot cope with this." % (i, s))
             r[i] = None
             lastdep = i
             lastver = ""