check for duplicate import blocks (#33190)

Importing to the same target address twice or importing the same ID
to multiple different resources of the same type is not allowed.

internal/command/testdata/validate-invalid/duplicate_import_ids/main.tf

@@ -0,0 +1,15 @@
+resource "aws_instance" "web" {
+}
+
+resource "aws_instance" "other_web" {
+}
+
+import {
+  to = aws_instance.web
+  id = "test"
+}
+
+import {
+  to = aws_instance.other_web
+  id = "test"
+}

internal/command/testdata/validate-invalid/duplicate_import_ids/output.json

@@ -0,0 +1,34 @@
+{
+  "format_version": "1.0",
+  "valid": false,
+  "error_count": 1,
+  "warning_count": 0,
+  "diagnostics": [
+    {
+      "severity": "error",
+      "summary": "Duplicate import for ID \"test\"",
+      "detail": "An import block for the ID \"test\" and a resource of type \"aws_instance\" was already declared at testdata/validate-invalid/duplicate_import_ids/main.tf:7,1-7. The same resource cannot be imported twice.",
+      "range": {
+        "filename": "testdata/validate-invalid/duplicate_import_ids/main.tf",
+        "start": {
+          "line": 12,
+          "column": 1,
+          "byte": 126
+        },
+        "end": {
+          "line": 12,
+          "column": 7,
+          "byte": 132
+        }
+      },
+      "snippet": {
+        "context": null,
+        "code": "import {",
+        "start_line": 12,
+        "highlight_start_offset": 0,
+        "highlight_end_offset": 6,
+        "values": []
+      }
+    }
+  ]
+}

internal/command/testdata/validate-invalid/duplicate_import_targets/main.tf

@@ -0,0 +1,12 @@
+resource "aws_instance" "web" {
+}
+
+import {
+  to = aws_instance.web
+  id = "test"
+}
+
+import {
+  to = aws_instance.web
+  id = "test2"
+}

internal/command/testdata/validate-invalid/duplicate_import_targets/output.json

@@ -0,0 +1,34 @@
+{
+  "format_version": "1.0",
+  "valid": false,
+  "error_count": 1,
+  "warning_count": 0,
+  "diagnostics": [
+    {
+      "severity": "error",
+      "summary": "Duplicate import configuration for \"aws_instance.web\"",
+      "detail": "An import block for the resource \"aws_instance.web\" was already declared at testdata/validate-invalid/duplicate_import_targets/main.tf:4,1-7. A resource can have only one import block.",
+      "range": {
+        "filename": "testdata/validate-invalid/duplicate_import_targets/main.tf",
+        "start": {
+          "line": 9,
+          "column": 1,
+          "byte": 85
+        },
+        "end": {
+          "line": 9,
+          "column": 7,
+          "byte": 91
+        }
+      },
+      "snippet": {
+        "context": null,
+        "code": "import {",
+        "start_line": 9,
+        "highlight_start_offset": 0,
+        "highlight_end_offset": 6,
+        "values": []
+      }
+    }
+  ]
+}