There are a number of tartans that are shared by more than one family or district.  The following tartans are identical:
Austin, Keith, and Marshall
Buchan and Cumming Hunting
Drummond of Strathallen and Ogilvie of Airlie
Galbraith, Hunter, Mitchell, and Russel
Melvile and Oliphant
MacDonald of Glencoe and MacIan
Prince of Wales and Duke of Rothsay Hunting
Campbell and Black Watch

There are also a number of tartans that are not available under one name but are woven under another.  For instance, the Kidd family tartan was adopted by the MacPhersons and is now sold as the red clan MacPherson tartan.  Likewise, the Spence or Spens tartan (which is not woven) is almost identical to the Drummond of Perth tartan (which is).
The Carolina District tartan is based on the Prince Charles Edward Stewart, and is almost identical to that tartan.  People seeking these tartans are advised to select the same tartans under the different names.

The following district tartans are sold today under these names:
Pertshire = Drummond of Perth
Argyll = Campbell of Cawdor
Atholl = Murray or Atholl
Eglinton = Montgomerie (not identical, but very similar)
Huntly = MacRae (red, not the hunting)
Sutherland = Black Watch

MASTER TARTAN LIST (click here for a quick image of any of these 9 oz tartans):
Abercrombie   M
Allison    M
Anderson   AM
Angus    A
Armstrong   AM
Austin    AM
Baird    AM
Barclay Hunting  A
Barclay Dress   M
Black Watch   AMW
Black Watch Dress  M
Blair    M
Borthwick   A
Borthwick Dress  A
Boyd    M
Brodie    M
Brodie Hunting  AM
Brown    M
Bruce    AM
Buchan   AM
Buchanan   AM
Buchanan Hunting  M
Buchanan (Old Sett)  AW
Burnett    A
Burns Check   M
Caledonia   A
Cameron   AM
Cameron of Erracht  AMW
Cameron Hunting  W
Cameron of Lochiel  AM
Campbell   AM
Campbell of Argyll  AMW
Campbell of Breadalbane AM
Campbell Faded  W
Campbell of Cawdor  AM
Campbell Dress  AM
Campbell of Louden  M
Carmichael   A
Carnegie   A
Chisolm   AM
Chisolm Hunting  AM
Christie   M
Clark    M
Clergy    M
Clergy (Green)  M
Cochrane   A
Cockburn   AM
Colquhoun   AM
Cooper    A
Craig    A
Cranston   M
Crawford   AM
Culloden   A
Cumming   M
Cumming Hunting  AMW
Cunningham   M
Dalziel    M
Davidson   AM
Davidson of Tulloch  A
Douglas   AM
Douglas Grey   AM
Drummond   M
Drummond of Perth  M
Drummond of Strathallen A
Dunbar  (family)  M
Duncan   AM
Dundas   M
Dundee   W
Earl of St. Andrews  M
Elliot    AM
Edinburgh   M
Erskine   M
Erskine Black&White  M
Farquharson   AMW
Ferguson   AMW
Fletcher   AM
Fletcher of Dunans  M
Flower of Scotland  A
Forbes    AM
Forsyth   AM
Fraser    AMW
Fraser Hunting  AM
Fraser of Lovat  M
Galbraith   AM
Galloway Red   M
Galloway Green  M
Gillies    A
Glasgow   M
Gordon   AMW
Gordon Dress   AM
Gordon Old   A
Gordon Red   A
Gow    M
Graham of Menteith  AMW
Graham of Montrose  AM
Grant    AMW
Gunn    AMW
Hamilton   AM
Hamilton Hunting  AM
Hay    AM
Henderson   AM
Holyrood   W
Home    AM
Hunter    AM
Inglis    A
Innes    AM
Inverness   A
Irvine (Irving, Irwin)  A
Jacobite   M
Johnston(e)   AM
Keith    AM
Kennedy   AMW
Kerr    AM
Kilgour   M
Lamont   AM
Lauder    A
Leatherneck (USMC)  M
Lennox   M
Leslie    M
Leslie Hunting   AM
Lindsay   AMW
Livingston   M
Logan & MacLennan  AM
Lochaber   W
Lumsden   M
MacAlister   M
MacAlpine   AM
MacArthur   AM
MacAuley   AM
MacAuley Hunting  A
MacBean   M
MacBeth   AM
MacCallum   AM
MacColl   M
MacDairmid   M
MacDonald Clan  AMW
MacDonald of Ardnamurchan  M
MacDonald of Boisdale M
MacDonald of Clanranald AM
MacDonald Dress  AM
MacDonald of Glencoe (MacIan) M
MacDonald of Glengarry AM
MacDonald of the Isles Green  AM
MacDonald, Lord of the Isles
 Hunting  A
MacDonald, Lord of the Isles
 Red   AM
MacDonald of Kingsburgh M
MacDonald of Sleat  M
MacDonald of Staffa  M
MacDonell of Keppoch AM
MacDougal   AM
MacDuff   AM
MacDuff Dress  AM
MacDuff Hunting  AM
MacEwan   AM
MacFarlane   AM
MacFarlane Black&White AM
MacFarlane Hunting  AM
MacGill   M
MacGillavry   M
MacGillavry Hunting  A
MacGregor   AM
MacGregor Hunting  A
MacGregor, Rob Roy  AM
MacHardy   A
MacIan   M
MacInnes   AM
MacIntosh   AM
MacIntosh Hunting  AMW
MacIntyre   AM
MacIntyre & Glenorchy M
MacIntyre Hunting  AMW
MacIver   M
MacKay   AMW
MacKay Blue   A
MacKellar   M
MacKenzie   AMW
MacKenzie Dress  AM
MacKillop   M
MacKinlay   A
MacKinnon   AM
MacKinnon Hunting  AM
MacLachlan   AMW
MacLachlan Hunting (old sett)  AM
MacLaine of Lochbuie M
MacLaine Hunting  A
MacLaren   AMW
MacLay (old sett)  M
MacLean of Duart  AM
MacLean Hunting  AM
MacLellan   A
MacLennan & Logan  AM
MacLeod of Lewis (dress) AM
MacLeod of Harris (hunting) AMW
MacMillan Dress  A
MacMillan Hunting  AM
MacMillan Old  AMW
MacNab   AM
MacNaughton   AM
MacNeil of Barra  AM
MacNeil of Colonsay  AM
MacPhail   M
MacPhail Hunting  A
MacPherson   AMW
MacPherson Dress  AM
MacPherson Hunting  AM
MacPhie   M
MacQuarrie   M
MacQueen   M
MacRae   AM
MacRae Dress   M
MacRae Hunting  AMW
MacSporran   A
MacTaggart   A
MacTavish   M
MacThomas   A
Malcolm   AM
Marshall   AM
Matheson   AM
Matheson Hunting  AM
Maxwell   M
Melville   A
Menzies Black&White AM
Menzies Green  AM
Middleton   M
Mitchell   AM
Montgomerie   AM
Moffat    M
Morgan   M
Morrison   AM
Morrison Red   AM
Mowat    A
Munro    AM
Murray of Atholl  AMW
Murray of Tulibardine  M
Musselburgh   A
Napier    AM
Nicolson   AM
Nicolson Hunting  AW
Nisbet    M
Ogilvie of Airlie  A
Ogilvie Hunting  AM
Oliphant   A
Princess Margaret Rose M
Prince of Wales  M
Ramsay   M
Ramsay Blue   A
Rattray    M
Rob Roy MacGregor  AM
Robertson   AMW
Robertson Hunting  AM
Rose    M
Rose Hunting   AM
Ross    AM
Ross Hunting   AM
Rothsay, Duke of  M
Rothsay, Duke of (Hunting) M
Roxburgh   A
Royal Canadian Air Force M
Russell    AM
Ruthven   M
Saint Patrick   M
Scott    AM
Scott Black&White  AM
Scott Brown   AM
Scott Green   AMW
Sir Walter Scott  M
Seton    M
Seton Hunting   M
Shaw    AM
Shaw Green   M
Shaw Red   M
Shepherd   M
Sinclair   AM
Sinclair Hunting  AM
Skene    M
Smith    AM
Stewart of Appin  AM
Stewart of Appin Hunting AM
Stewart of Atholl  M
Stewart Black   M
Stuart of Bute   M
Stewart Dress   AM
Stewart Hunting  AMW
Stewart Muted Blue  W
Stewart Old   AMW
Stewart, Prince Charles Edward  M
Stewart Royal   AM
Stirling & Bannockburn M
Sutherland Old  AMW
Tara (Murphy)   M
Tartan Army   M
Taylor    W
Thomson   M
Thomson Hunting  M
Thomson Camel  M
Urquhart   AM
Urquhart Broad Red  A
Wallace   AMW
Wallace Hunting  A
Watson   A
Weir    A
Wemyss   M
Wilson    A

CANADIAN TARTANS:
Cape Breton   A
British Columbia  M
Maple Leaf   W
New Brunswick  M
Newfoundland   A
Nova Scotia   M
Prince Edward Island  M