diff --git a/bits/56_dirtree.js b/bits/56_dirtree.js
index 4a1a093..d23d961 100644
--- a/bits/56_dirtree.js
+++ b/bits/56_dirtree.js
@@ -1,19 +1,25 @@
 	var now = new Date(1987, 1, 19), j = 0;
+	// Track which names exist
+	var fullPaths = Object.create ? Object.create(null) : {};
 	var data/*:Array<[string, CFBEntry]>*/ = [];
 	for(i = 0; i < cfb.FullPaths.length; ++i) {
+		fullPaths[cfb.FullPaths[i]] = true;
 		if(cfb.FileIndex[i].type === 0) continue;
 		data.push([cfb.FullPaths[i], cfb.FileIndex[i]]);
 	}
 	for(i = 0; i < data.length; ++i) {
 		var dad = dirname(data[i][0]);
-		s = false;
-		for(j = 0; j < data.length; ++j) if(data[j][0] === dad) s = true;
-		if(!s) data.push([dad, ({
-			name: filename(dad).replace("/",""),
-			type: 1,
-			clsid: HEADER_CLSID,
-			ct: now, mt: now,
-			content: null
-		}/*:any*/)]);
+		s = fullPaths[dad];
+		if(!s) {
+			data.push([dad, ({
+				name: filename(dad).replace("/",""),
+				type: 1,
+				clsid: HEADER_CLSID,
+				ct: now, mt: now,
+				content: null
+			}/*:any*/)]);
+			// Add name to set
+			fullPaths[dad] = true;
+		}
 	}
 
diff --git a/cfb.flow.js b/cfb.flow.js
index b755124..d884851 100644
--- a/cfb.flow.js
+++ b/cfb.flow.js
@@ -756,22 +756,28 @@ function rebuild_cfb(cfb/*:CFBContainer*/, f/*:?boolean*/)/*:void*/ {
 	if(!gc && !f) return;
 
 	var now = new Date(1987, 1, 19), j = 0;
+	// Track which names exist
+	var fullPaths = Object.create ? Object.create(null) : {};
 	var data/*:Array<[string, CFBEntry]>*/ = [];
 	for(i = 0; i < cfb.FullPaths.length; ++i) {
+		fullPaths[cfb.FullPaths[i]] = true;
 		if(cfb.FileIndex[i].type === 0) continue;
 		data.push([cfb.FullPaths[i], cfb.FileIndex[i]]);
 	}
 	for(i = 0; i < data.length; ++i) {
 		var dad = dirname(data[i][0]);
-		s = false;
-		for(j = 0; j < data.length; ++j) if(data[j][0] === dad) s = true;
-		if(!s) data.push([dad, ({
-			name: filename(dad).replace("/",""),
-			type: 1,
-			clsid: HEADER_CLSID,
-			ct: now, mt: now,
-			content: null
-		}/*:any*/)]);
+		s = fullPaths[dad];
+		if(!s) {
+			data.push([dad, ({
+				name: filename(dad).replace("/",""),
+				type: 1,
+				clsid: HEADER_CLSID,
+				ct: now, mt: now,
+				content: null
+			}/*:any*/)]);
+			// Add name to set
+			fullPaths[dad] = true;
+		}
 	}
 
 	data.sort(function(x,y) { return namecmp(x[0], y[0]); });
diff --git a/cfb.js b/cfb.js
index 7882e20..a3b5550 100644
--- a/cfb.js
+++ b/cfb.js
@@ -738,22 +738,28 @@ function rebuild_cfb(cfb, f) {
 	if(!gc && !f) return;
 
 	var now = new Date(1987, 1, 19), j = 0;
+	// Track which names exist
+	var fullPaths = Object.create ? Object.create(null) : {};
 	var data = [];
 	for(i = 0; i < cfb.FullPaths.length; ++i) {
+		fullPaths[cfb.FullPaths[i]] = true;
 		if(cfb.FileIndex[i].type === 0) continue;
 		data.push([cfb.FullPaths[i], cfb.FileIndex[i]]);
 	}
 	for(i = 0; i < data.length; ++i) {
 		var dad = dirname(data[i][0]);
-		s = false;
-		for(j = 0; j < data.length; ++j) if(data[j][0] === dad) s = true;
-		if(!s) data.push([dad, ({
-			name: filename(dad).replace("/",""),
-			type: 1,
-			clsid: HEADER_CLSID,
-			ct: now, mt: now,
-			content: null
-		})]);
+		s = fullPaths[dad];
+		if(!s) {
+			data.push([dad, ({
+				name: filename(dad).replace("/",""),
+				type: 1,
+				clsid: HEADER_CLSID,
+				ct: now, mt: now,
+				content: null
+			})]);
+			// Add name to set
+			fullPaths[dad] = true;
+		}
 	}
 
 	data.sort(function(x,y) { return namecmp(x[0], y[0]); });
diff --git a/xlscfb.flow.js b/xlscfb.flow.js
index fefebdf..9991d94 100644
--- a/xlscfb.flow.js
+++ b/xlscfb.flow.js
@@ -604,22 +604,28 @@ function rebuild_cfb(cfb/*:CFBContainer*/, f/*:?boolean*/)/*:void*/ {
 	if(!gc && !f) return;
 
 	var now = new Date(1987, 1, 19), j = 0;
+	// Track which names exist
+	var fullPaths = Object.create ? Object.create(null) : {};
 	var data/*:Array<[string, CFBEntry]>*/ = [];
 	for(i = 0; i < cfb.FullPaths.length; ++i) {
+		fullPaths[cfb.FullPaths[i]] = true;
 		if(cfb.FileIndex[i].type === 0) continue;
 		data.push([cfb.FullPaths[i], cfb.FileIndex[i]]);
 	}
 	for(i = 0; i < data.length; ++i) {
 		var dad = dirname(data[i][0]);
-		s = false;
-		for(j = 0; j < data.length; ++j) if(data[j][0] === dad) s = true;
-		if(!s) data.push([dad, ({
-			name: filename(dad).replace("/",""),
-			type: 1,
-			clsid: HEADER_CLSID,
-			ct: now, mt: now,
-			content: null
-		}/*:any*/)]);
+		s = fullPaths[dad];
+		if(!s) {
+			data.push([dad, ({
+				name: filename(dad).replace("/",""),
+				type: 1,
+				clsid: HEADER_CLSID,
+				ct: now, mt: now,
+				content: null
+			}/*:any*/)]);
+			// Add name to set
+			fullPaths[dad] = true;
+		}
 	}
 
 	data.sort(function(x,y) { return namecmp(x[0], y[0]); });
diff --git a/xlscfb.js b/xlscfb.js
index ef2fa71..ba81cd4 100644
--- a/xlscfb.js
+++ b/xlscfb.js
@@ -570,22 +570,28 @@ function rebuild_cfb(cfb, f) {
 	if(!gc && !f) return;
 
 	var now = new Date(1987, 1, 19), j = 0;
+	// Track which names exist
+	var fullPaths = Object.create ? Object.create(null) : {};
 	var data = [];
 	for(i = 0; i < cfb.FullPaths.length; ++i) {
+		fullPaths[cfb.FullPaths[i]] = true;
 		if(cfb.FileIndex[i].type === 0) continue;
 		data.push([cfb.FullPaths[i], cfb.FileIndex[i]]);
 	}
 	for(i = 0; i < data.length; ++i) {
 		var dad = dirname(data[i][0]);
-		s = false;
-		for(j = 0; j < data.length; ++j) if(data[j][0] === dad) s = true;
-		if(!s) data.push([dad, ({
-			name: filename(dad).replace("/",""),
-			type: 1,
-			clsid: HEADER_CLSID,
-			ct: now, mt: now,
-			content: null
-		})]);
+		s = fullPaths[dad];
+		if(!s) {
+			data.push([dad, ({
+				name: filename(dad).replace("/",""),
+				type: 1,
+				clsid: HEADER_CLSID,
+				ct: now, mt: now,
+				content: null
+			})]);
+			// Add name to set
+			fullPaths[dad] = true;
+		}
 	}
 
 	data.sort(function(x,y) { return namecmp(x[0], y[0]); });