diff --git a/recipes.py b/recipes.py
index ed68506..b2bb808 100644
--- a/recipes.py
+++ b/recipes.py
@@ -1,5 +1,6 @@
 from abc import abstractmethod
 from collections import defaultdict
+import collections
 import sys
 from typing import Dict, List, Any, Optional, Set
 import os
@@ -10,6 +11,38 @@ import jinja2
 import jsonschema
 import jsonschema.exceptions
 
+ISSUE_UNKNOWN_UNIT = "unknown-unit"
+ISSUE_UNKNOWN_INGREDIENT = "unknown-ingredient"
+ISSUE_DUPLICATE_UNITS = "duplicate-units"
+ISSUE_KNOWN_PRICE_UNKNOWN_CONVERSION = "known-price-unknown-conversion"
+
+class Issue:
+    def __init__(self, id: str, msg: str) -> None:
+        self.id = id
+        self.msg = msg
+
+class Issues:
+    def __init__(self) -> None:
+        self.errors: List[Issue] = []
+        self.warnings: List[Issue] = []
+
+    def error(self, id: str, msg: str) -> None:
+        self.errors.append(Issue(id, msg))
+
+    def warn(self, id: str, msg: str) -> None:
+        self.warnings.append(Issue(id, msg))
+
+    def check(self) -> int:
+        retcode = len(self.errors) != 0
+
+        for msg in self.errors:
+            print(f"ERROR {msg.id}: {msg.msg}")
+        for msg in self.warnings:
+            print(f"WARNING {msg.id}: {msg.msg}")
+
+        self.errors.clear()
+        self.warnings.clear()
+        return retcode
 
 class Context:
     def __init__(self) -> None:
@@ -17,7 +50,7 @@ class Context:
         self.default_unit = Unit(self, {"name": "piece"})
         self.units.units.append(self.default_unit)
         self.ingredients = Ingredients(self)
-        self.issues: List[str] = []
+        self.issues = Issues()
 
 
 class Element:
@@ -103,12 +136,20 @@ class Units:
                 return unit
         return None
 
+    def validate(self) -> None:
+        unitnames = []
+        for unit in self.units:
+            unitnames.append(unit["name"])
+        for unitname, num in collections.Counter(unitnames).items():
+            if num>1:
+                self.ctx.issues.error(ISSUE_DUPLICATE_UNITS, f"units.yaml: {unitname} should only have one entry, found {num}")
+
 
 class Ingredient(Element):
     def load(self, dct: Dict[str, Any]) -> None:
         if "prices" in dct:
             pricedb = PriceDBs(self.ctx)
-            self.ctx.issues += pricedb.load(dct["prices"])
+            pricedb.load(dct["prices"])
             self["prices"] = pricedb
 
         conversions = []
@@ -155,8 +196,8 @@ class Ingredient(Element):
         # find path between conversions
         path = self.dfs(conversions, unitfrom["name"], unitto["name"])
         if path is None:
-            print(
-                f'WARNING: {self["name"]} has a known price, but conversion {unitfrom["name"]} -> {unitto["name"]} not known'
+            self.ctx.issues.warn(ISSUE_KNOWN_PRICE_UNKNOWN_CONVERSION,
+                f'{self["name"]} has a known price, but conversion {unitfrom["name"]} -> {unitto["name"]} not known'
             )
             return None
         assert len(path) != 0
@@ -210,11 +251,10 @@ class PriceDBs:
         self.ctx = ctx
         self.pricedbs: List[PriceDB] = []
 
-    def load(self, lst: List[Any]) -> List[str]:
+    def load(self, lst: List[Any]) -> None:
         for elem in lst:
             pricedb = PriceDB(self.ctx, elem)
             self.pricedbs.append(pricedb)
-        return self.ctx.issues
 
     def __repr__(self) -> str:
         return self.pricedbs.__repr__()
@@ -229,7 +269,7 @@ class PriceDB(Element):
             unitstr = dct["unit"]
             self["unit"] = self.ctx.units.get(unitstr)
             if self["unit"] is None:
-                self.ctx.issues.append(f"unknown unit {unitstr}")
+                self.ctx.issues.error(ISSUE_UNKNOWN_UNIT, f"unknown unit {unitstr}")
         else:
             self["unit"] = self.ctx.default_unit
 
@@ -238,7 +278,7 @@ class IngredientInstance(Element):
     def load(self, dct: Dict[str, Any]) -> None:
         ingredient = self.ctx.ingredients.get(dct["name"])
         if ingredient is None:
-            self.ctx.issues.append(f"unknown ingredient {dct['name']}")
+            self.ctx.issues.error(ISSUE_UNKNOWN_INGREDIENT, f"unknown ingredient {dct['name']}")
         self["ingredient"] = ingredient
 
         if "amount" not in dct:
@@ -248,7 +288,7 @@ class IngredientInstance(Element):
             unitstr = dct["unit"]
             self["unit"] = self.ctx.units.get(unitstr)
             if self["unit"] is None:
-                self.ctx.issues.append("unknown unit {unitstr}")
+                self.ctx.issues.error(ISSUE_UNKNOWN_UNIT, "unknown unit {unitstr}")
         else:
             self["unit"] = self.ctx.default_unit
 
@@ -356,18 +396,17 @@ class Builder:
         unitsschema = self.load_file("schemas/units.json")
         jsonschema.validate(instance=unitsdct, schema=unitsschema)
         self.ctx.units.load(unitsdct)
-        if len(self.ctx.issues) != 0:
-            for issue in self.ctx.issues:
-                print("ERROR in units.yaml:", issue)
+        retcode = self.ctx.issues.check()
+        if retcode != 0:
             return 1
+        self.ctx.units.validate()
 
         ingredientsdct = self.load_file(dir + "/ingredients.yaml")
         ingredientsschema = self.load_file("schemas/ingredients.json")
         jsonschema.validate(instance=ingredientsdct, schema=ingredientsschema)
         self.ctx.ingredients.load(ingredientsdct)
-        if len(self.ctx.issues) != 0:
-            for issue in self.ctx.issues:
-                print("ERROR in ingredients.yaml:", issue)
+        retcode = self.ctx.issues.check()
+        if retcode != 0:
             return 1
 
         return 0
@@ -389,11 +428,12 @@ class Builder:
             recipe = Recipe(self.ctx, recipedct)
             recipe.srcpath = file
             recipe.outpath = file[:-5] + ".html"
+            if self.ctx.issues.check() != 0:
+                continue
             recipes.append(recipe)
 
-        if len(self.ctx.issues) != 0:
-            for issue in self.ctx.issues:
-                print("ERROR", issue)
+        retcode = self.ctx.issues.check()
+        if retcode != 0:
             return 1
 
         self.rendertemplate(