fix(themes): Handle shared parent theme with component shadowing

If gatsby-theme-a and gatsby-theme-b both install and use
gatsby-theme-c (as a parent theme) this would cause an error
to be raised when either sibling theme shadowed a component
from gatsby-theme-c when it should be ignored.

packages/gatsby/src/internal-plugins/webpack-theme-component-shadowing/index.js

@@ -1,6 +1,7 @@
 const path = require(`path`)
 const debug = require(`debug`)(`gatsby:component-shadowing`)
 const fs = require(`fs`)
+const _ = require(`lodash`)
 
 module.exports = class GatsbyThemeComponentShadowingResolverPlugin {
   cache = {}
@@ -14,10 +15,16 @@ module.exports = class GatsbyThemeComponentShadowingResolverPlugin {
   apply(resolver) {
     resolver.plugin(`relative`, (request, callback) => {
       // find out which theme's src/components dir we're requiring from
-      const matchingThemes = this.themes.filter(name =>
+      const allMatchingThemes = this.themes.filter(name =>
         request.path.includes(path.join(name, `src`))
       )
-      // 0 matching themes happens a lot fo rpaths we don't want to handle
+
+      // Two sibling themes can each have the same parent theme, when
+      // this occurs it shouldn't cause an error because when shadowing
+      // because they'll typically resolve to the same module.
+      const matchingThemes = _.uniq(allMatchingThemes)
+
+      // 0 matching themes happens a lot for paths we don't want to handle
       // > 1 matching theme means we have a path like
       //   `gatsby-theme-blog/src/components/gatsby-theme-something/src/components`
       if (matchingThemes.length > 1) {