diff --git a/lib/www/server/lib/comparisons/geometric-differences.js b/lib/www/server/lib/comparisons/geometric-differences.js
index bed21d1..2f9f06d 100644
--- a/lib/www/server/lib/comparisons/geometric-differences.js
+++ b/lib/www/server/lib/comparisons/geometric-differences.js
@@ -439,6 +439,14 @@ async function saveGroup (group, opts = {}) {
 
 		for (const [ baselineProjectID, monitorProjectID ] of combinations(pids, 2)) {
 			try {
+				if (!opts.overwrite) {
+					const exists = await get(baselineProjectID, monitorProjectID);
+					if (exists) {
+						DEBUG("Not overwriting existing comparison between %s and %s. Skipping", baselineProjectID, monitorProjectID);
+						continue;
+					}
+				}
+
 				const isSaved = await save(baselineProjectID, monitorProjectID);
 				if (isSaved) {
 					await saveSample(baselineProjectID, monitorProjectID, opts.sampleOpts);