fix duplicate module preload

Author: mbostock

src/render.ts

@@ -131,12 +131,12 @@ function getImportPreloads(parseResult: ParseResult, path: string): Iterable<str
   if (inputs.has("dot")) specifiers.add("npm:@viz-js/viz");
   if (inputs.has("mermaid")) specifiers.add("npm:mermaid").add("npm:d3");
   if (inputs.has("tex")) specifiers.add("npm:katex");
-  const preloads: string[] = [];
+  const preloads = new Set<string>();
   for (const specifier of specifiers) {
-    preloads.push(resolveImport(specifier));
+    preloads.add(resolveImport(specifier));
   }
   if (parseResult.cells.some((cell) => cell.databases?.length)) {
-    preloads.push("/_observablehq/database.js");
+    preloads.add("/_observablehq/database.js");
   }
   return preloads;
 }

test/input/build/imports/foo/foo.js

@@ -1,3 +1,4 @@
+import "https://cdn.jsdelivr.net/npm/d3/+esm";
 import {bar} from "../bar/bar.js";
 
 export const foo = "foo";

test/input/build/imports/foo/foo.md

@@ -1,6 +1,7 @@
 # Foo
 
 ```js
+import * as d3 from "npm:d3";
 import {bar} from "../bar/bar.js";
 
 display(bar);

test/output/build/imports/_import/foo/foo.js

@@ -1,3 +1,4 @@
+import "https://cdn.jsdelivr.net/npm/d3/+esm";
 import {bar} from "../bar/bar.js";
 
 export const foo = "foo";

test/output/build/imports/foo/foo.html

@@ -6,25 +6,27 @@
 <link rel="stylesheet" type="text/css" href="https://fonts.googleapis.com/css2?family=Source+Serif+Pro:ital,wght@0,400;0,600;0,700;1,400;1,600;1,700&display=swap">
 <link rel="stylesheet" type="text/css" href="/_observablehq/style.css">
 <link rel="modulepreload" href="/_observablehq/runtime.js">
+<link rel="modulepreload" href="https://cdn.jsdelivr.net/npm/d3/+esm">
 <link rel="modulepreload" href="/_import/bar/bar.js">
 <link rel="modulepreload" href="/_import/bar/baz.js">
 <link rel="modulepreload" href="/_import/foo/foo.js">
 <script type="module">
 
 import {define} from "/_observablehq/client.js";
 
-define({id: "d54e931e", inputs: ["display"], outputs: ["bar"], body: async (display) => {
+define({id: "a9220fae", inputs: ["display"], outputs: ["d3","bar"], body: async (display) => {
+const d3 = await import("https://cdn.jsdelivr.net/npm/d3/+esm");
 const {bar} = await import("/_import/bar/bar.js");
 
 display(bar);
-return {bar};
+return {d3,bar};
 }});
 
 </script>
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 <main id="observablehq-main" class="observablehq">
 <h1 id="foo" tabindex="-1"><a class="observablehq-header-anchor" href="#foo">Foo</a></h1>
-<div id="cell-d54e931e" class="observablehq observablehq--block"></div>
+<div id="cell-a9220fae" class="observablehq observablehq--block"></div>
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