Variables: a CSV header that names one variable twice is refused

dataset.import read two header cells that named one variable (City
and City, or Shown and #Shown) into the same column, so each row kept
only the last non-blank cell and the other value was dropped without a
word. The import is now refused with the two column numbers, and the
message has a row in every complete-message catalog.

crates/engine/src/cmd/variablecmds.rs

@@ -731,7 +731,7 @@ pub fn specs() -> Vec<CommandSpec> {
             "Import Data Sets…",
             ["Window", "Variables"],
             None,
-            "{path? | data? | dataBase64? | csv?} load datasets from CSV, comma- or tab-separated (header: name, then variable names; one row per dataset; without the name column every header is a variable and the rows take the first free names Data Set 1, 2…). A header that names no variable defines one, unbound: `#Name` defines a visibility variable, `@Name` defines an image variable, and a header without a prefix defines a text variable. A prefixed header whose name is a variable of another kind, and a `%` header that isn't a variable's exact name (graph data files), are refused, and so is a header that would define a variable while the variables are locked. Rows whose name already exists are replaced → {imported, updated, names, defined}",
+            "{path? | data? | dataBase64? | csv?} load datasets from CSV, comma- or tab-separated (header: name, then variable names; one row per dataset; without the name column every header is a variable and the rows take the first free names Data Set 1, 2…). A header that names no variable defines one, unbound: `#Name` defines a visibility variable, `@Name` defines an image variable, and a header without a prefix defines a text variable. A prefixed header whose name is a variable of another kind, and a `%` header that isn't a variable's exact name (graph data files), are refused, and so are two headers that name one variable and a header that would define a variable while the variables are locked. Rows whose name already exists are replaced → {imported, updated, names, defined}",
             has_doc,
             dataset_import
         ),