Add README, fix column deduplication in parser, remove .env from git

src/sqlmem/parser.py

@@ -63,9 +63,13 @@ def _extract_table(statement: exp.Select) -> str:
 
 
 def _extract_columns(statement: exp.Select) -> list[str]:
-    columns = []
+    seen: set[str] = set()
+    columns: list[str] = []
     for col in statement.find_all(exp.Column):
-        columns.append(col.name)
+        name = col.name
+        if name not in seen:
+            seen.add(name)
+            columns.append(name)
     if not columns:
         raise UnsupportedQueryError("Could not extract column names from query.")
     return columns