parser: add duplicate import symbol detect (fix #25185) (#25187)

vlib/v/fmt/tests/import_duplicate_input.vv

@@ -1,18 +0,0 @@
-import math
-import os
-import math
-// keep comment
-import gg
-import gg { MouseButton }
-import time { Duration }
-import time { Duration }
-import math.complex { Complex }
-import math.complex { Complex }
-
-const mypi = math.pi
-const mb = MouseButton{}
-const complex = Complex{}
-
-fn main() {
-	println(os.path_separator)
-}

vlib/v/parser/module.v

@@ -310,6 +310,11 @@ fn (mut p Parser) import_syms(mut parent ast.Import) {
 	for p.tok.kind == .name {
 		pos := p.tok.pos()
 		alias := p.check_name()
+		if alias in p.imported_symbols {
+			p.error_with_pos('cannot register symbol `${alias}`, it was already imported',
+				pos)
+			return
+		}
 		p.imported_symbols[alias] = parent.mod + '.' + alias
 		// so we can work with this in fmt+checker
 		parent.syms << ast.ImportSymbol{

vlib/v/parser/tests/module_import_same_symbol2_err.out

@@ -0,0 +1,3 @@
+vlib/v/parser/tests/module_import_same_symbol2_err.vv:1:20: error: cannot register symbol `max`, it was already imported
+    1 | import math { max, max }
+      |                    ~~~

vlib/v/parser/tests/module_import_same_symbol2_err.vv

@@ -0,0 +1 @@
+import math { max, max }

vlib/v/parser/tests/module_import_same_symbol_err.out

@@ -0,0 +1,4 @@
+vlib/v/parser/tests/module_import_same_symbol_err.vv:2:17: error: cannot register symbol `max`, it was already imported
+    1 | import math { max }
+    2 | import arrays { max }
+      |                 ~~~

vlib/v/parser/tests/module_import_same_symbol_err.vv

@@ -0,0 +1,2 @@
+import math { max }
+import arrays { max }