diff --git a/alv/registry.moon b/alv/registry.moon
index c41e684..a39a1d7 100644
--- a/alv/registry.moon
+++ b/alv/registry.moon
@@ -17,22 +17,22 @@ class Registry
 
   --- set the current registration.
   --
-  -- @tparam string\number index the registration index
+  -- @tparam Tag tag the Tag to register
   -- @tparam any expr the registration value
   -- @tparam[default=false] boolean ignore_dup ignore duplicate registrations
-  register: (index, expr, ignore_dup=false) =>
-    L\trace "reg: setting #{index} to #{expr}"
-    if not ignore_dup and @map[index]
-      error Error 'tag', "duplicate tags [#{index}]!"
-    @map[index] = expr
-
-  --- request identity and registration for blank tag.
-  --
-  -- @tparam Tag tag the blank tag
-  -- @tparam any expr the registration value
-  init: (tag, expr) =>
-    L\trace "reg: init pending to #{expr}"
-    table.insert @pending, { :tag, :expr }
+  register: (tag, expr, ignore_dup=false) =>
+    index = tag\index!
+
+    if index and (not @map[index] or ignore_dup)
+      L\trace "reg: setting #{index} to #{expr}"
+      @map[index] = expr
+    else
+      if index
+        L\warn "duplicate tag [#{index}], reassigning repeated occurance"
+        tag\set nil
+      else
+        L\trace "reg: init #{tag} to #{expr}"
+      table.insert @pending, { :tag, :expr }
 
 --- members
 -- @section members
@@ -45,7 +45,6 @@ class Registry
   -- All calls go `begin_eval` must be matched with either a call to
   -- `end_eval` or `rollback_eval`.
   begin_eval: =>
-    @latest_map = @last_map
     @begin_tick!
     @map, @pending = {}, {}
 
diff --git a/alv/tag.moon b/alv/tag.moon
index 904cd20..d7becab 100644
--- a/alv/tag.moon
+++ b/alv/tag.moon
@@ -37,11 +37,7 @@ class Tag
   -- Will mark blank tags for auto-assignment at the end of the eval cycle.
   --
   -- @tparam any expr the value to register
-  register: (expr) =>
-    if index = @index!
-      Registry.active!\register index, expr
-    else
-      Registry.active!\init @, expr
+  register: (expr) => Registry.active!\register @, expr
 
   --- create a copy of this tag scoped to a `parent` tag.
   --
@@ -52,10 +48,7 @@ class Tag
   clone: (parent) =>
     -- ensure this tag is registered for the current eval cycle,
     -- even if it is blank and has no associated value
-    if index = @index!
-      Registry.active!\register index, dummy, true
-    else
-      Registry.active!\init @, dummy
+    Registry.active!\register @, dummy, true
 
     assert parent, "need parent to clone!"
     ClonedTag @, parent
@@ -77,9 +70,13 @@ class Tag
   index: => @value
 
   --- callback to set value for blank tags.
-  -- @tparam number value
+  --
+  -- `value` may be blank to reassign duplicate tags.
+  --
+  -- @tparam ?number value
   set: (value) =>
-    assert not @value, "#{@} is not blank"
+    either_or = (@value or value) and not (@value and value)
+    assert either_or, "unexpected :set #{value} on #{@}"
     @value = value
 
 --- static functions