diff --git a/api/src/org/labkey/api/exp/api/ExperimentService.java b/api/src/org/labkey/api/exp/api/ExperimentService.java
index df052bbe503..e775fd6d834 100644
--- a/api/src/org/labkey/api/exp/api/ExperimentService.java
+++ b/api/src/org/labkey/api/exp/api/ExperimentService.java
@@ -613,9 +613,23 @@ static void validateParentAlias(Map<String, String> aliasMap, Set<String> reserv
                 throw new IllegalArgumentException(String.format("Parent alias header is reserved: %1$s", trimmedKey));
             }
 
-            if (updatedDomainDesign != null && !existingAliases.contains(trimmedKey) && updatedDomainDesign.getFieldByName(trimmedKey) != null)
+            if (updatedDomainDesign != null)
             {
-                throw new IllegalArgumentException(String.format("An existing " + dataTypeNoun + " property conflicts with parent alias header: %1$s", trimmedKey));
+                if (!existingAliases.contains(trimmedKey))
+                {
+                    var field = updatedDomainDesign.getFieldByName(trimmedKey);
+                    if (field != null)
+                    {
+                        throw new IllegalArgumentException(String.format("An existing %1$s property conflicts with parent alias header: %2$s", dataTypeNoun, trimmedKey));
+                    }
+                }
+                // GH Issue 1257: If there are conflicts with import aliases, this should be an error since it produces ambiguity during import
+                var field = updatedDomainDesign.getFieldByImportAlias(trimmedKey);
+                if (field != null)
+                {
+                    throw new IllegalArgumentException(String.format("Field %1$s has an import alias %2$s that conflicts with a parent alias header.", field.getName(), trimmedKey));
+                }
+
             }
 
             if (!dupes.add(trimmedKey))
diff --git a/api/src/org/labkey/api/exp/property/DomainUtil.java b/api/src/org/labkey/api/exp/property/DomainUtil.java
index d10d3d5a854..2f443345214 100644
--- a/api/src/org/labkey/api/exp/property/DomainUtil.java
+++ b/api/src/org/labkey/api/exp/property/DomainUtil.java
@@ -26,6 +26,7 @@
 import org.labkey.api.assay.AbstractAssayProvider;
 import org.labkey.api.collections.CaseInsensitiveHashMap;
 import org.labkey.api.collections.CaseInsensitiveHashSet;
+import org.labkey.api.collections.CaseInsensitiveLinkedHashMap;
 import org.labkey.api.collections.IntHashMap;
 import org.labkey.api.collections.LongHashMap;
 import org.labkey.api.data.ColumnInfo;
@@ -1563,6 +1564,9 @@ public static ValidationException validateProperties(@Nullable Domain domain, @N
         Set<String> reservedPrefixes = (null != domain && null != domainKind) ? domainKind.getReservedPropertyNamePrefixes() : updates.getReservedFieldNamePrefixes();
         Map<String, Integer> namePropertyIdMap = new CaseInsensitiveHashMap<>();
         Map<String, String> altNameMap = new CaseInsensitiveHashMap<>();
+        // GH Issue 1257: import alias -> the fields declaring it.
+        Map<String, List<GWTPropertyDescriptor>> importAliasMap = new CaseInsensitiveLinkedHashMap<>();
+        Set<String> fieldNames = new CaseInsensitiveHashSet();
         ValidationException exception = new ValidationException();
         Map<Integer, String> propertyIdNameMap = getOriginalFieldPropertyIdNameMap(orig);//key: orig property id, value : orig field name
 
@@ -1577,6 +1581,15 @@ public static ValidationException validateProperties(@Nullable Domain domain, @N
                 continue;
             }
 
+            fieldNames.add(name);
+
+            // GH Issue 1257: Collect alias to field list mapping
+            for (String alias : new CaseInsensitiveHashSet(ColumnRenderPropertiesImpl.convertToSet(field.getImportAliases())))
+            {
+                if (!alias.equalsIgnoreCase(name)) // an alias that repeats the field's name is redundant but not ambiguous, so skip it
+                    importAliasMap.computeIfAbsent(alias, k -> new ArrayList<>()).add(field);
+            }
+
             if (ILLEGAL_PROPERTY_NAMES.contains(name.trim()))
             {
                 exception.addError(new SimpleValidationError(getDomainErrorMessage(updates, "The field name '" + name + "' is not allowed.")));
@@ -1674,9 +1687,28 @@ public static ValidationException validateProperties(@Nullable Domain domain, @N
             }
         }
 
+        // GH Issue 1257: import aliases must be unique across the domain and must not collide with a field name, since
+        // ImportAliasable.Helper.createImportMap() resolves names, labels, and aliases into one case-insensitive map.
+        importAliasMap.forEach((alias, fields) -> {
+            String names = fields.stream().map(GWTPropertyDescriptor::getName).sorted().collect(Collectors.joining(", "));
+
+            if (fields.size() > 1)
+                addImportAliasErrors(exception, updates, fields, "Duplicate import alias " + alias + " for fields " + names + ".");
+
+            if (fieldNames.contains(alias))
+                addImportAliasErrors(exception, updates, fields, "Import alias " + alias + " on field" + (fields.size() == 1 ? " " : "s ") + names + " conflicts with a field name.");
+        });
+
         return exception;
     }
 
+    /** Anchor an import alias error on every field that declares the offending alias so the designer can highlight them. */
+    private static void addImportAliasErrors(ValidationException exception, GWTDomain<?> updates, List<GWTPropertyDescriptor> fields, String message)
+    {
+        for (GWTPropertyDescriptor field : fields)
+            exception.addError(new PropertyValidationError(getDomainErrorMessage(updates, message), field.getName(), field.getPropertyId()));
+    }
+
     @Nullable
     private static Map<Integer, String> getOriginalFieldPropertyIdNameMap(@Nullable GWTDomain<?> orig)
     {
diff --git a/api/src/org/labkey/api/gwt/client/model/GWTDomain.java b/api/src/org/labkey/api/gwt/client/model/GWTDomain.java
index a0bc62bb98a..58eebeb2b1a 100644
--- a/api/src/org/labkey/api/gwt/client/model/GWTDomain.java
+++ b/api/src/org/labkey/api/gwt/client/model/GWTDomain.java
@@ -19,6 +19,8 @@
 import com.fasterxml.jackson.annotation.JsonIgnore;
 import lombok.Getter;
 import lombok.Setter;
+import org.labkey.api.collections.CaseInsensitiveHashSet;
+import org.labkey.api.data.ColumnRenderPropertiesImpl;
 import org.labkey.api.gwt.client.DefaultValueType;
 
 import java.util.ArrayList;
@@ -197,6 +199,17 @@ public FieldType getFieldByName(String name)
         return null;
     }
 
+    public FieldType getFieldByImportAlias(String alias)
+    {
+        for (FieldType field : getFields(true))
+        {
+            Set<String> importAliases = ColumnRenderPropertiesImpl.convertToSet(field.getImportAliases());
+            if (new CaseInsensitiveHashSet(importAliases).contains(alias))
+                return field;
+        }
+        return null;
+    }
+
     /**
      * @return Indicates that the property can't be removed from the domain. The property may or may not be nullable.
      */