diff --git a/docs/manpage.rst b/docs/manpage.rst
index 2ee154353..81d50ad0b 100644
--- a/docs/manpage.rst
+++ b/docs/manpage.rst
@@ -2377,4 +2377,4 @@ This is simply a copy of what you should see on running **man mlr** at a command
     
     
     
-    				  2020-11-15			     MILLER(1)
+    				  2020-11-24			     MILLER(1)
diff --git a/docs/manpage.txt b/docs/manpage.txt
index d1bc71eec..58c923339 100644
--- a/docs/manpage.txt
+++ b/docs/manpage.txt
@@ -2367,4 +2367,4 @@ SEE ALSO
 
 
 
-				  2020-11-15			     MILLER(1)
+				  2020-11-24			     MILLER(1)
diff --git a/docs/mlr.1 b/docs/mlr.1
index 8751fae69..b3626fb80 100644
--- a/docs/mlr.1
+++ b/docs/mlr.1
@@ -2,12 +2,12 @@
 .\"     Title: mlr
 .\"    Author: [see the "AUTHOR" section]
 .\" Generator: ./mkman.rb
-.\"      Date: 2020-11-15
+.\"      Date: 2020-11-24
 .\"    Manual: \ \&
 .\"    Source: \ \&
 .\"  Language: English
 .\"
-.TH "MILLER" "1" "2020-11-15" "\ \&" "\ \&"
+.TH "MILLER" "1" "2020-11-24" "\ \&" "\ \&"
 .\" -----------------------------------------------------------------
 .\" * Portability definitions
 .\" ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
diff --git a/go/cases-to-do.txt b/go/cases-to-do.txt
index f0523decb..879227932 100644
--- a/go/cases-to-do.txt
+++ b/go/cases-to-do.txt
@@ -21,7 +21,12 @@ rrv ./reg-test/cases/case-c-dsl-triple-for-loops.sh
   "Continuation statements are split into the final boolean, and the statements before (if any)."
 
 rrv ./reg-test/cases/case-c-label.sh
-* needs uniqueness-checker
+* handles old/new clashes differently from C impl :^/
+
+================================================================
+ENV
+
+rrv ./reg-test/cases/case-c-dsl-functional-tests.sh
 
 ================================================================
 MULTIPLE ISSUES:
@@ -43,18 +48,11 @@ rrv ./reg-test/cases/case-c-multi-character-ixs.sh
 rrv ./reg-test/cases/case-c-implicit-header-csv-input.sh
 rrv ./reg-test/cases/case-c-missing-final-lf.sh
 
-================================================================
-ENV
-
-rrv ./reg-test/cases/case-c-dsl-functional-tests.sh
-
 ================================================================
 PUT -S / -F / -A
 
-rrv ./reg-test/cases/case-c-dsl-type-inference.sh
 rrv -C1 ./reg-test/cases/case-c-dsl-for-srec-loops.sh
 rrv -C1 ./reg-test/cases/case-c-dsl-type-inference.sh
-rrv ./reg-test/cases/case-c-dsl-pythonic-division.sh
 
 rrv -C20 ./reg-test/cases/case-c-dsl-local-map-variable-write-read.sh
 rrv -C20 ./reg-test/cases/case-c-dsl-local-map-variable-write-read.sh
@@ -63,6 +61,8 @@ rrv -C20 ./reg-test/cases/case-c-dsl-local-map-variable-write-read.sh
 ================================================================
 ARRAY/MAP
 
+rrv ./reg-test/cases/case-c-localvar-unsets.sh
+
 rrv -C5 ./reg-test/cases/case-c-dsl-for-oosvar-loops.sh
 rrv ./reg-test/cases/case-c-dsl-mapvar-assignments.sh
 
@@ -108,16 +108,6 @@ rrv ./reg-test/cases/case-c-dsl-trailing-commas.sh
 rrv ./reg-test/cases/case-c-dsl-argpass-typedecl.sh
 rrv ./reg-test/cases/case-c-dsl-multipart-scripts.sh
 
-================================================================
-FORMATTING?
-
-rrv ./reg-test/cases/case-c-dsl-forbind-typedecl.sh
-rrv ./reg-test/cases/case-c-dsl-indirect-srec-assignments.sh
-rrv ./reg-test/cases/case-c-dsl-operator-associativity.sh
-rrv ./reg-test/cases/case-c-dsl-operator-precedence.sh
-rrv ./reg-test/cases/case-c-dsl-typed-overlay.sh
-rrv ./reg-test/cases/case-c-localvar-unsets.sh
-
 ================================================================
 PARTIALLY IMPLEMENTED:
 
@@ -222,19 +212,3 @@ rrv ./reg-test/cases/case-c-step.sh
 rrv ./reg-test/cases/case-c-top.sh
 rrv ./reg-test/cases/case-c-uniq.sh
 rrv ./reg-test/cases/case-c-utf8-bom.sh
-
-================================================================
-JUST FORMATTING NOW??
-
-rrv ./reg-test/cases/case-c-dsl-type-inference.sh
-rrv -C1 ./reg-test/cases/case-c-dsl-for-srec-loops.sh
-rrv -C1 ./reg-test/cases/case-c-dsl-type-inference.sh
-rrv ./reg-test/cases/case-c-dsl-pythonic-division.sh
-
-rrv -C20 ./reg-test/cases/case-c-dsl-local-map-variable-write-read.sh
-rrv -C1 ./reg-test/cases/case-c-dsl-local-map-variable-write-read.sh
-
-================================================================
-rrv -C20 ./reg-test/cases/case-c-dsl-empty-statements.sh
-rrv -C1 ./reg-test/cases/case-c-dsl-empty-statements.sh
-================================================================
diff --git a/go/src/miller/mappers/label.go b/go/src/miller/mappers/label.go
index 56701a4f1..3e2461425 100644
--- a/go/src/miller/mappers/label.go
+++ b/go/src/miller/mappers/label.go
@@ -1,6 +1,7 @@
 package mappers
 
 import (
+	"errors"
 	"flag"
 	"fmt"
 	"os"
@@ -60,9 +61,13 @@ func mapperLabelParseCLI(
 
 	argi += 1
 
-	mapper, _ := NewMapperLabel(
+	mapper, err := NewMapperLabel(
 		newNames,
 	)
+	if err != nil {
+		fmt.Fprint(os.Stderr, err)
+		return nil
+	}
 
 	*pargi = argi
 	return mapper
@@ -95,6 +100,20 @@ type MapperLabel struct {
 func NewMapperLabel(
 	newNames []string,
 ) (*MapperLabel, error) {
+	uniquenessChecker := make(map[string]bool)
+	for _, newName := range newNames {
+		_, ok := uniquenessChecker[newName]
+		if ok {
+			return nil, errors.New(
+				fmt.Sprintf(
+					"mlr label: labels must be unique; got duplicate \"%s\"\n",
+					newName,
+				),
+			)
+		}
+		uniquenessChecker[newName] = true
+	}
+
 	this := &MapperLabel{
 		newNames: newNames,
 	}