diff --git a/lua/oil/fs.lua b/lua/oil/fs.lua
index 8e325c3..774b0a0 100644
--- a/lua/oil/fs.lua
+++ b/lua/oil/fs.lua
@@ -3,6 +3,8 @@ local M = {}
 ---@type boolean
 M.is_windows = vim.loop.os_uname().version:match("Windows")
 
+M.is_mac = vim.loop.os_uname().sysname == "Darwin"
+
 ---@type string
 M.sep = M.is_windows and "\\" or "/"
 
diff --git a/lua/oil/mutator/parser.lua b/lua/oil/mutator/parser.lua
index 5edddb7..a20f3c0 100644
--- a/lua/oil/mutator/parser.lua
+++ b/lua/oil/mutator/parser.lua
@@ -1,5 +1,6 @@
 local cache = require("oil.cache")
 local columns = require("oil.columns")
+local fs = require("oil.fs")
 local util = require("oil.util")
 local view = require("oil.view")
 local FIELD = require("oil.constants").FIELD
@@ -104,7 +105,7 @@ M.parse = function(bufnr)
   local adapter = util.get_adapter(bufnr)
   if not adapter then
     table.insert(errors, {
-      lnum = 1,
+      lnum = 0,
       col = 0,
       message = string.format("Cannot parse buffer '%s': No adapter", bufname),
     })
@@ -121,6 +122,18 @@ M.parse = function(bufnr)
       original_entries[child[FIELD.name]] = child[FIELD.id]
     end
   end
+  local seen_names = {}
+  local function check_dupe(name, i)
+    if fs.is_mac or fs.is_windows then
+      -- mac and windows use case-insensitive filesystems
+      name = name:lower()
+    end
+    if seen_names[name] then
+      table.insert(errors, { message = "Duplicate filename", lnum = i - 1, col = 0 })
+    else
+      seen_names[name] = true
+    end
+  end
   for i, line in ipairs(lines) do
     if line:match("^/%d+") then
       local parsed_entry, entry, err = M.parse_line(adapter, line, column_defs)
@@ -143,11 +156,12 @@ M.parse = function(bufnr)
         end
         table.insert(errors, {
           message = message,
-          lnum = i,
+          lnum = i - 1,
           col = 0,
         })
         goto continue
       end
+      check_dupe(parsed_entry.name, i)
       local meta = entry[FIELD.meta]
       if original_entries[parsed_entry.name] == parsed_entry.id then
         if entry[FIELD.type] == "link" and (not meta or meta.link ~= parsed_entry.link_target) then
@@ -187,7 +201,7 @@ M.parse = function(bufnr)
       if vim.startswith(name, "/") then
         table.insert(errors, {
           message = "Paths cannot start with '/'",
-          lnum = i,
+          lnum = i - 1,
           col = 0,
         })
         goto continue
@@ -200,6 +214,7 @@ M.parse = function(bufnr)
           entry_type = "link"
           name, link = unpack(link_pieces)
         end
+        check_dupe(name, i)
         table.insert(diffs, {
           type = "new",
           name = name,
diff --git a/tests/mutator_spec.lua b/tests/mutator_spec.lua
index 55077a3..dfed196 100644
--- a/tests/mutator_spec.lua
+++ b/tests/mutator_spec.lua
@@ -108,6 +108,41 @@ a.describe("mutator", function()
       }, errors)
     end)
 
+    it("errors on duplicate names", function()
+      vim.cmd.edit({ args = { "oil-test:///foo/" } })
+      local bufnr = vim.api.nvim_get_current_buf()
+      set_lines(bufnr, {
+        "foo",
+        "foo/",
+      })
+      local _, errors = parser.parse(bufnr)
+      assert.are.same({
+        {
+          message = "Duplicate filename",
+          lnum = 2,
+          col = 0,
+        },
+      }, errors)
+    end)
+
+    it("errors on duplicate names for existing files", function()
+      local file = cache.create_and_store_entry("oil-test:///foo/", "a.txt", "file")
+      vim.cmd.edit({ args = { "oil-test:///foo/" } })
+      local bufnr = vim.api.nvim_get_current_buf()
+      set_lines(bufnr, {
+        "a.txt",
+        string.format("/%d a.txt", file[FIELD.id]),
+      })
+      local _, errors = parser.parse(bufnr)
+      assert.are.same({
+        {
+          message = "Duplicate filename",
+          lnum = 2,
+          col = 0,
+        },
+      }, errors)
+    end)
+
     it("ignores new dirs with empty name", function()
       vim.cmd.edit({ args = { "oil-test:///foo/" } })
       local bufnr = vim.api.nvim_get_current_buf()